# Splitting a canister filter between two small tanks



## Klinckman (Dec 1, 2013)

to keep water level in each tank correct, the only easy (cheap) way is to have the filter take water from 1 tank and output the filtered water to the other tank and between the two tanks have a U shaped intake tube siphoning from 1 tank to the other.


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## PlantedRich (Jul 21, 2010)

It could be done but with some hazards involved. One problem is what might happen if the intake on one got clogged more than the other. As time went on with one intake not drawing the full share of water but with the water coming back into that tank remaining the same, you could be setting up so that one tank gets too full and one too low. A flood could be the result? Tying the two together with a siphon works until the siphon gets a leaf over it. Same result then. You could wind up with one 5 trying to hold 8 and the other holding 2. 
I don't recommend that.


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## Kudaria (Jan 7, 2013)

Yea, sounds like a bad idea...darn I wish they made small filters with good flow control. As it is I just went with two small aquatop nano HOB filters instead.


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## scapegoat (Jun 3, 2010)

Klinckman said:


> to keep water level in each tank correct, the only easy (cheap) way is to have the filter take water from 1 tank and output the filtered water to the other tank and between the two tanks have a U shaped intake tube siphoning from 1 tank to the other.


that isn't even a good idea. your filter and siphon will need to have the same flow rate. and a filters rate also depends on how much crap is in it.


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## PlantedRich (Jul 21, 2010)

I once tried daisy chaining five tanks together for automatic water changing. I was aware of the problems if the siphon was lost but just not aware of what it takes to maintain the siphon. It was in a basement so I went with it. Someway water has air in it that collects in the siphon tube. I spent way too much time just checking which tube had filled with enough air to not work.


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## mark546 (Sep 12, 2013)

Why not just put a ball valve in the output?


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## Kudaria (Jan 7, 2013)

How do the fish stores get it to work? Surely they don't run a filter per tank?


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## jeffdenney (Jun 21, 2013)

Large sump setups with overflows and ball valve to control flow.


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## PlantedRich (Jul 21, 2010)

As long as there is somebody on hand to monitor it and a concrete floor, it works better. But then a large volume system makes it work better. But the ball valve to control flow only helps as long as the flow stay uniform. When we use the size plumbing most of us use, a single leaf in the wrong spot can throw the whole thing into turmoil. Sure a guy can work around that by adding more and more backup systems but then it gets into whether it is worth the whole deal. Most reach a point where they go another way.


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## scotty b (Oct 23, 2012)

if they are empty , you can find a place that drills tanks locally and conect them with bulkheads so they maintain water levels


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## Legot (Jun 29, 2013)

That^

It essentially makes two tanks into one, with no siphons to worry about. Just put a screen/mesh to keep everything in it's own tank and you're golden. The larger diameter bulkhead/pipe combo you use, the exponentially greater reduction in flow.


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## Kudaria (Jan 7, 2013)

Unfortunately they aren't empty any more, but if the small HOB's don't work well then I may break them down and connect them with bulkheads as suggested. What size diameter would you suggest for a 4-5 gallon betta tank? I'm about to order some glass cutting bits from bulk reef supply this week to cut a hole for a bulkhead drain in a terrarium anyway so I might as well get a full selection.


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## Texan78 (Nov 17, 2013)

Kudaria said:


> What size diameter would you suggest for a 4-5 gallon betta tank?


1/2"

It would make this even better and probably safer if you could use a sump with this set up if you go with bulkheads hard plumbed with PVC and ball valves to control flow independently to each tank with dual pumps from the sump. You could get away with using a canister but I think it would be a little more trickier when it comes to controlling flow as you're relying on a single canister which has a restricted intake and return based on a single contained pump. A canister intakes water at the same rate it is returning it based on the rated contained pump. You direct plumb bulkheads to the canister it will now intake water faster than it can possibly pump out which may result in flooding the canister. Just something to consider.


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## scapegoat (Jun 3, 2010)

you'd really set up a sump system for a 4-5 gallon tank? w/ two pumps? come on...

and you can't flood a canister... it won't take in any more water than it physically is able to. It works on a closed loop and only takes in as much water as it pumps out. you turn off the pump and water stops flowing.


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## Texan78 (Nov 17, 2013)

scapegoat said:


> you'd really set up a sump system for a 4-5 gallon tank? w/ two pumps? come on...
> 
> and you can't flood a canister... it won't take in any more water than it physically is able to. It works on a closed loop and only takes in as much water as it pumps out. you turn off the pump and water stops flowing.


No, but I wouldn't be putting bulk heads in a 4-5G tank ether. Read the comments above. Bulkheads are designed mainly for sumps, not canister filters. When there is more water flowing into the canister that it can pump out you will flood it and blow the o-ring sealing it. 

That is why I suggested a sump if he considers drilling the tank and adding bulkheads as he is asking about. Otherwise no, me personally I wouldn't be using a sump but I also wouldn't be wasting time putting bulkheads in a 4-5 gallon tank but who are we to tell others what they can and can't do.


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## scapegoat (Jun 3, 2010)

Texan78 said:


> No, but I wouldn't be putting bulk heads in a 4-5G tank ether. Read the comments above. Bulkheads are designed mainly for sumps, not canister filters. When there is more water flowing into the canister that it can pump out you will flood it and blow the o-ring sealing it.
> 
> That is why I suggested a sump if he considers drilling the tank and adding bulkheads as he is asking about. Otherwise no, me personally I wouldn't be using a sump but I also wouldn't be wasting time putting bulkheads in a 4-5 gallon tank but who are we to tell others what they can and can't do.


but a canister filter isn't going to get more water flowing into it than it can pump out. I've never heard of a canister filter blowing. especially not with a 4-5g tank.

I think they were talking about bulkheads to attach the tanks to each other horizontally so that the inlet of the canister can be in one tank while the outlet is in another. the water level will normalize between the two tanks. still not something I'd do.

but yeah, you're not blowing out a canister filter unless there is some extreme example you're thinking of. perhaps a big multi-hundred gallon tank on one floor of the house with the canister below it. I can understand the water pressure building up enough from there.


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## Texan78 (Nov 17, 2013)

scapegoat said:


> but a canister filter isn't going to get more water flowing into it than it can pump out.


If it is used with an overflow it will and if the power goes out you're screwed.

If they were talking about connecting it horizontally then that is a completely different story. 

There were others suggesting sumps with overflows as well. I was simply pointing out _*IF*_ he went that route don't use it with a canister. Yet somehow my post was singled out and questioned... :icon_roll

And yes, you can blow out a canister filter. I have seen it many, many, many times, more times than you think under several different circumstances out of their normal operation. Sometimes even under normal operation when the return gets clogged the filter will still pump water in. When you do the line of work I do you come across a lot of not so bright people.


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## AndreyT (Apr 28, 2011)

Texan78 said:


> A canister intakes water at the same rate it is returning it based on the rated contained pump. You direct plumb bulkheads to the canister it will now intake water faster than it can possibly pump out which may result in flooding the canister. Just something to consider.


That makes no sense whatsoever.

Canister always intakes the same amount of water it returns. It is ensured by a simple physical principle directly related to the principle of "conjoined vessels". It is absolutely unrelated to the pump. You can put absolutely any pump in there, you can remove the pump altogether, you can use intake and outflow tubes of absolutely any (and different) diameter. The canister will always automatically intake the same amount of water it returns.

As long as the canister remains sealed, it is absolutely impossible to break that principle, regardless of what you do. And no, there's no such issue as "flooding" the canister. The canister is always fully flooded - that is the fundamental principle behind its operation.

You must be thinking about wet-dry filter, whose operation is relying on the precise balance between inflow and outflow. Canister filter is a fundamentally different system, which has nothing in common with wet-dry filters.


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## xmas_one (Feb 5, 2010)

Kudaria said:


> How do the fish stores get it to work? Surely they don't run a filter per tank?


Actually, a properly set up LFS will have a separate filter for each tank. It's the bonehead ones that run centralized filters.


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## Texan78 (Nov 17, 2013)

AndreyT said:


> Canister always intakes the same amount of water it returns.


Intaking the same amount of water it returns Yes, UNLESS, you increase the intake which is more than it can return. Water going into the pump is still limited by how much water can be pumped out. If you increase the flow into the canister which is more than it can be pumped out it will "Blow out" as some call it and flood. I understand it is flooded internally under normal operation, but when it intakes more water than it can hold it is now flooded beyond it's normally working operation.


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## scapegoat (Jun 3, 2010)

Texan78 said:


> Intaking the same amount of water it returns Yes, UNLESS, you increase the intake which is more than it can return. Water going into the pump is still limited by how much water can be pumped out. If you increase the flow into the canister which is more than it can be pumped out it will "Blow out" as some call it and flood. I understand it is flooded internally under normal operation, but when it intakes more water than it can hold it is now flooded beyond it's normally working operation.


you haven't done a thing to explain what causes a canister to intake more than it can exhaust. and you're telling this to a forum full of folks that use canister filters and not one has corroborated your story.


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## AndreyT (Apr 28, 2011)

Texan78 said:


> Intaking the same amount of water it returns Yes, UNLESS, you increase the intake which is more than it can return.


What does "you increase the intake" mean? Install a additioonal pump at the intake to force the water in?

In any case, no. Anything you do will simply increase circulation. In a cansiter filter, it is not possible to "increase the intake" without automatically inscreasing the outflow by the same amount.



Texan78 said:


> Water going into the pump is still limited by how much water can be pumped out.


That is false. 

In a canister filter water is not "pumped out". Canister filter is an inherently balanced system. In a canister filter the pressure of incoming water is always perfectly balanced by the pressure of outflowing water. This is just basic physics of "сommunicating vessels". For this reason, the pump in a cansiter filter does not perform any work directed at "pumping water out", i.e. it does not perform any work agains gravity. The pump in the cansiter filter is there to encourage the water to circulate, not to work against gravity.

This is the fundamental principle that defined the very existence of canister filters. The were created specifically to take advantage of this principle. 

This is the fundamental difference between wet-dry and canister filters. In wet-dry filters the pump has to "pump water out" against gravity. But in canister filter pump does not "feel" any gravity at all.



Texan78 said:


> If you increase the flow into the canister which is more than it can be pumped out it will "Blow out" as some call it and flood. I understand it is flooded internally under normal operation, but when it intakes more water than it can hold it is now flooded beyond it's normally working operation.


Well, if you attach a full-size water pump from a fire truck to the canister filter intake and make it to force the water into the cansiter, then yes, it will definitely blow out! Although the hose will probably blow out before the canister.

But it is impossible to make it "blow out" by any _passive_ change. Regardles of how you shape the intake, it will not make the canister to blow out.

P.S. Anothr way to make the canister blow out is to exceed the rated height difference between the aquarium and the canister. But this is not the issue in our case.


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## Legot (Jun 29, 2013)

You've obviously put too much thought into that response Andrey, I don't see why...

Ok, the canister flooding thing, technically it is possible, but only if the seals are pretty worn/poorly made. And it would be a heavy leak at the worst. If the seals are good, the only way is to actually add a pump, pushing into the intake of the canister faster than it can handle. It would be difficult to do, and not very likely, but it's possible.

I like sumps! If I was willing to drill my nice desk I'd have a 20g sump on my upcoming 5g chi, saltwater guys do larger sumps than displays more often than you'd think. If you've got the space, go for a sump, much simpler than running a canister on two tanks, especially on this scale. It can make the system so much bigger (volume wize) and will generally increase your lifespan.


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## Legot (Jun 29, 2013)

Oh, thought of another way to blow out a canister (without attaching it to a fire truck...........)

If your intake is placed deeper in the tank than your return, there is some additional pressure within the canister. If that distance was significant it will most certainly cause a premature filter leak.


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## AndreyT (Apr 28, 2011)

Texan78 said:


> And yes, you can blow out a canister filter. I have seen it many, many, many times, more times than you think under several different circumstances out of their normal operation. Sometimes even under normal operation when the return gets clogged the filter will still pump water in. When you do the line of work I do you come across a lot of not so bright people.


Er... You probably misinterpreted the reason for the blow out.

The number one reason for canister filter "blow out" is exceeding the maximum height difference between the aquarium and the canister. The pressure inside the filter is defined primarily by the height difference between the aquarium and the canister. The seals that keep the canister watertight are rated for certain pressure (with some safety margin). If that pressure is exceeded, the canister will begin to leak.

This is actually the reason canister filters have limited height difference (and not the need to "pump water up" as is often incorreclty believed.)

Some not-so-bright people install their canisters exceeding the maximum height difference, thus exceeding the maximum pressure inside the canister. Such canister might "work fine" at first, while seals still hold. But it can burst after any seemingly insignificant event. It can burst just because the outflow got clogged. It can burst just becuse someone sneezed right next to it.

But it has absolutely nothing to do with any intake or outflow modifications, as you seem to incorrectlty believe. It is not possible to blow a canister filter by any passive changes to intake or outflow.


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## nofearengineer (Mar 20, 2013)

Yes, while a canister can't magically create more water at the return than at the intake, the pressures at the two locations are still different. 

They'd better be, otherwise the water wouldn't flow in two opposite directions (relative to the canister of course). And that difference in pressure isn't due to head.

If you want to know how much (and your tank isn't near anything valuable :hihi just lift your return hose until you get zero flow. Measure the height above the water surface, and voila! You've got your max. pump head (approx...no friction or fitting losses included). Multiply height inches by .036 and you've got your pump psi value.

Assuming you haven't just 'sploded your canister...:icon_twis


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## AndreyT (Apr 28, 2011)

nofearengineer said:


> Yes, while a canister can't magically create more water at the return than at the intake, the pressures at the two locations are still different.


Well, since the water is flowing agains significant resistance (media, drag etc.) then, of course, somewhere we must have a difference in pressure. That difference is indeed created by the pump. That's what the pump is for.

However, the important part is that in a canister filter that difference is defined only by the drag (in the media and in the tubes). It is not affected by the height difference. Canister filter is a system with _zero static head_. (Or with almoist zero static head, if one uses an elevated spraybar.)



nofearengineer said:


> If you want to know how much (and your tank isn't near anything valuable :hihi just lift your return hose until you get zero flow. Measure the height above the water surface, and voila! You've got your max. pump head (approx...no friction or fitting losses included). Multiply height inches by .036 and you've got your pump psi value.


Yes, that way you will calculate how much "spare capacity" your pump has. Technically, you can use that capacity to lift water, i.e. pump water from one tank with lower water level to another tank with higher water level. However, canister filters are not designed for such use. Canister filters are supposed to be used with pefectly balanced input and output syphons, i.e. the intake tube is supposed to get water from the same water volume as the one the outflow tube returns water to. I.e. both tubes are supposed to "see" the same water level. That way the pump will not have to work against any static head. It will work against media and tube drag only.

Forcing that weak impeller pump in a cansiter filter to work against significant static head (i.e. forcing it to actually pump the water up against gravity) will probably significantly reduce the lifetime of the pump. If you ever saw a return pump in a wet-dry filter (i.e. a pump that actually has to work against gravity all the time), you know that it uses a completely different design.


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## lochaber (Jan 23, 2012)

I really think that going with two HOB is probably the better choice.

Otherwise, you would have to put in some siphons, bulkheads, or have one tank flow into the other water-fall style.

Any of those methods is going to have all sorts of potential problems (leaks, siphon breaks, etc.). These problems can be addressed and worked around, but it would take quite a bit of effort/money for tanks of that size. I just really don't think it's worth it.


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## Legot (Jun 29, 2013)

Seeing as this is a much more controversial topic than I thought it was originally, the HOBs would probably be the simplest/cheapest option in the long run...


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## scotty b (Oct 23, 2012)

I would conect the tanks with bulkheads, there are a lot of salt water tanks ive seen thith multiple 20 or 50 gallons attached together with bulkheads no issues at all


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