# Fluval 306 Canister filter review



## PlantedRich (Jul 21, 2010)

Be aware of some danger points and it may last a long time.
I find that seal to be a major question due to design. It puts a lot of pressure on a pretty thin area of the head and that can cause a crack that requires replacing the head. Fatal for mine as the cost is near the price of other filters? I recommend doing all the preventative stuff possible and one of those is keeping the top O-ring well lubed to avoid any excess pressure there.


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## AndreyT (Apr 28, 2011)

shawnwaldon said:


> It is rated for 300 GPH and I calculated it at 150 GPH at a height of approximately 4'.


Peformance of a canister filer does not in any way depend on the height. It depends only on the obstructivity (length, diameter, bends etc.) of the connecting hoses. So, it would be more useful if you described your hoses. Height does not matter at all (unless you detected leaks).


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## shawnwaldon (Dec 5, 2013)

I will keep an eye on that gasket, thank you for the heads up. 

AndreyT head height always matters for every pump, that is why they put different ratings for GPH based on head height on every pump. If you take my 4' feet of hose and lay it horizontal with the outlet of the filter it will have a higher flow rate because there is no down ward force against it. If you take that same 4' and put it vertical (example would be up to your tank) then there is a down ward force the pump is working against which reduces the GPH the pump can produce. But to answer you inquiry about how my hoses are routed they are flexable hose that has a slight bend from the filter out of the stand then another bend over the aquarium rim. None of the bends are as sharp as a pvc fitting so the effect on the flow is low.


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## AndreyT (Apr 28, 2011)

shawnwaldon said:


> AndreyT head height always matters for every pump, that is why they put different ratings for GPH based on head height on every pump.


"Head height" is one of the important standard characteristics of every pump, which describes how strong the pump is. It is indeed important in this role.

However, canister filter is a *closed system*. In a closed system gravitational static head is always *zero*. There's no gravitational "head height" the pump has to work against in a canister filter. This feature of closed systems is exactly why we even have canister filters: they were invented specifically to take advantage of *zero static head* property of closed systems. This is why canister filters can employ relatively weak, small and quiet impeller pumps. And they don't need positive displacement pumps. All these pumps have to do it to work against friction in the hoses and resistance of filter media. Canister filter pump never has to work against gravity (!). As far as canister filter's pump is concerned, gravity does not exist.



shawnwaldon said:


> If you take my 4' feet of hose and lay it horizontal with the outlet of the filter it will have a higher flow rate because there is no down ward force against it. If you take that same 4' and put it vertical (example would be up to your tank) then there is a down ward force the pump is working against which reduces the GPH the pump can produce.


Absolutely false. Regardless of how you orient you hose, the GPH will always remain constant. 

To explain it in simple terms, the "downforce" in one (intake) hose of your canister filter is always _precisely counterbalanced_ by the "downforce" in the other (output) hose. The net sum of these two "downforces" is always precisely *zero*. This is the reason why the pump in canister filter never sees any "downforce" and never has to work against any "downforce". This is why, contrary to your belief, placement of your canister will not affects GPH at all.

Again, canister filters are completely insensitive to the height difference. Regardless of how high your tank is (as long as hose length remains constant), canister filter will always maintain constant GPH. Canister filter has absolutely ho height limit as far as pump power is concerned. This is the fundamental physical principle that governs operation of closed hydraulic systems, like canister filters. 

The real reason canister filter actually have height limits has nothing to do with pump power, head height, "downforce" or GPH. The reason is completely different: the greater the height difference between your filter and your tank - the greater the *water pressure* inside your canister. The rubber seals in your filer are rated only to certain pressure level. If you put your filter too low, the increased pressure will cause the seals to leak. Every canister filter has a height difference limit, which is purely *pressure limited*.

Theoretically, if you somehow reinforce your canister body, the seals and the hoses to make sure they can withstand the pressure, you can put your fish tank on top of The Empire State Building and put your canister filter in the basement. And you will still get about the same GPH from its original small pump. Just keep in mind that longer hoses produce more friction, and more friction means less GPH. This friction will reduce GPH, but not your "downforce". There's no flow-reducing "downforce" in canister filters.


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## shawnwaldon (Dec 5, 2013)

All though I would love to get into a debate with you and present actual proven facts in an attempt to prove our points I don't have time. I have very little available time being a truck driver. We both have backed our individual beliefs with what we presume to be proof so I guess we will have to agree to disagree.


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## AndreyT (Apr 28, 2011)

shawnwaldon said:


> All though I would love to get into a debate with you and present actual proven facts in an attempt to prove our points I don't have time. I have very little available time being a truck driver. We both have backed our individual beliefs with what we presume to be proof so I guess we will have to agree to disagree.


Fair enough. 

But maybe you should consider the bigger picture: if you managed to show the world a canister filter that would so blatantly defy established laws of physics as you describe (those "actual proven facts"), you'd turn modern science upside down and propel the humanity further down the road of knowledge. Besides, there might be a pie in it for you. Wouldn't it be worth taking some time off those truck driving duties?


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## PlantedRich (Jul 21, 2010)

I will not try to vote on this in either direction. Been there done that and never ends. But there may be a point that is real simple being missed. 
The water coming down does pretty well offset the weight of the water going up, making head a pretty small factor in this case. 
One way to look at this is how small the motor on filters compared to pumps which do actually work against head pressure. Filters are able to use tiny motors as all they need to do is boost the water a couple inches over the tank side and let the siphon do the real work. If we took the head off a filter and put it by itself in a bucket, I doubt it could pump high enough to get out of the bucket! 
But then I do eat dinner with truck drivers lots of times and I argue with engineers all the time, so I pretty well know, neither will change .


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## AndreyT (Apr 28, 2011)

PlantedRich said:


> The water coming down does pretty well offset the weight of the water going up, making head a pretty small factor in this case.


If the water coming down did not _exactly_ balance the water going up, you would be able to unplug your filter pump and observe water to flow around in circles by itself forever - a classic _perpetuum mobile_  The laws of physics require that two connected siphons feeding from the same body of water balance each other _precisely_.

A setup with a canister filter _might_ have a non-zero static head. That would be a setup where the output end of the hose is _elevated_ above the water surface of the tank (for example, a spray bar that drops water into the tank from some height). But even in this case the static head is defined only by the difference between water level and spray bar level. It does not in any way depend on how low the canister itself is placed under the tank. If the spray bar is installed 1 inch above the water surface, then canister pump will "see" exactly 1 inch of static head, regardless of where the canister itself is located.

But if the output end is _submerged_, the static head is always exactly zero.


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## PlantedRich (Jul 21, 2010)

That's why i rarely volunteer to discuss things with engineers unless I'm being paid well!!!


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## Rogozhin75 (Aug 15, 2017)

PlantedRich said:


> That's why i rarely volunteer to discuss things with engineers unless I'm being paid well!!!


I'm an engineer and I only use the socratic method on myself if my wife is paying me.


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## shawnwaldon (Dec 5, 2013)

Ok so I can't help but feel this thread is wildly veering off course with what I intended to accomplish with it. It was purely intended to be a product review to give what I felt was useful information for someone that was looking to purchase the filter. I only gave the information I had procured though my experience with the filter. I know when I am doing research on filters there are three main things I want to know and look for. 

1: Dependability

2: In use flow compared to rated flow

3: Ease of use

I never intended this thread to be a discussion about the scientific definition of flow. One thing I do know is, when I am doing research on a product and a thread seems to go off topic for a period of time then I don't proceed into the many other pages of the thread. I find that many threads do this and it is a shame because who knows how much good information has been undiscovered for this very reason. My intent for this thread is to give people a chance to put in their experiences and ask questions about this product. I will never tell people to not feel free to post what they want, but I will respectfully ask that we stay on topic with my thread. If you guys want to discuss the merits of the science of water flow please create a different thread. Oh and for the record if us truckers took a break from our trucker duties then life as you know it would stop in a hurry. Just remember most everything you have was brought to where you got it by a truck.


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