# Can Magnesium become toxic to fish?



## plakat (Mar 2, 2008)

Pretty much anything can be toxic if you have enough of it. I think you need a lot of magnesium to OD on it thought. 

Most tap water should have sufficient levels of Mg, Ca, and Na for your fish. Some people do add additional amounts because of plants but they aren't thought of as algae preventatives. Having the right balance of all nutrients will suppress or inhibit the growth of algae.


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## ER9 (Aug 2, 2008)

thats what i thought as well but i'm suffering another terrible b/g algae outbreak now in my second tank. iv'e read everything on this stuff on all the popular fishkeeping/ plant websites. they all regurgitate the same info like they are all copying and pasting text from each other. its frustrating to say the least because none of the few reasons given are problems my tank suffers. iv'e been through this before. something else entirely is going on.

unsatisfied i did more of my own research away from the fish sites and came upon some interesting information regarding fish health/algae growth (specifically cyano bacteria). there is evidence that specific spectrum of lighting and certain water conditions, heavier in magnesium, calcium and electrolytes can be not only beneficial to fish health but suppresive to b/g algae. all i have is magnesium and aquarium salt onhand so i thought i might experiment, which i did start last night. will be watching my b/g algae very carefully to see any possitive effects.

only problem is i really have no way to measure these disolved minerals in my aquarium except for Kh so i'm kind of blindly feeling my way along. just dont want to overdo anything like magnesium one day and come home to a tank full of floating fish.


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## Left C (Nov 15, 2003)

ER9 said:


> only problem is i really have no way to measure these dissolved minerals in my aquarium except for KH so i'm kind of blindly feeling my way along. just dont want to overdo anything like magnesium one day and come home to a tank full of floating fish.


You can use the fertilator to calculate your Ca and Mg additions.

You need a GH test kit and not a KH test kit to measure the hardness of the water when adding GH Booster, Equilibrium, Mg, Ca, etc.
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4345+13526&pcatid=13526

You can use a Ca test kit to find out how much Ca and Mg is in the water with the info from the GH test kit.
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4345+4469&pcatid=4469

A Conductivity Monitor is a precision instrument for the measurement of hardness in freshwater. 
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4485

Sometimes a (TDS) total dissolved solids meter is mentioned. Hanna makes a DiST® 5 that offers a 3-in-1 solution that instantly tests conductivity, total dissolved solids (TDS), and temperature.
http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=9848

"What is TDS?"
http://www.dieselduck.net/machine/07 controls/2004 The use of TDS.pdf

KH measures the alkalinity or buffering capacity of the water.


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## ER9 (Aug 2, 2008)

wow leftC. thank you.


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## Left C (Nov 15, 2003)

You are very welcome and I hope this helps you, ER9.

Good luck on your testing.


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## ER9 (Aug 2, 2008)

Left C said:


> Sometimes a (TDS) total dissolved solids meter is mentioned. Hanna makes a DiST® 5 that offers a 3-in-1 solution that instantly tests conductivity, total dissolved solids (TDS), and temperature.
> http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=9848


even more interestingly the ph/orp tester measures redox potential which is the end result of all this effort. wow i had no idea this was already common enough practice companies would devote resources to make these testers.


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## Left C (Nov 15, 2003)

There are many different types of water testing equipment on the market to test:
freshwater aquariums, rivers, wells, springs, lakes, etc., drinking water, sewage, treatment plants, etc. and 
saltwater/brackish seas, oceans, bays, aquariums, treatment plants, etc.

There are some very good books written by Dr. Charles E. Boyd at Auburn University that may be helpful to you. He is a professor in the Department of Fisheries and Allied Aquaculture. The one that I have is named: Water Quality in Ponds for Aquaculture. You can check it out at most Universities and Community Colleges. It may be very helpful to you.

Water Quality in Ponds for Aquaculture: http://www.amazon.com/s/ref=nb_ss_g...er+Quality+in+Ponds+for+Aquaculture&x=10&y=21

:


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## KDahlin (Mar 12, 2007)

You can add quite a lot of magnesium before it reaches toxic levels. At higher doses it acts as a mild laxative on fish and as an analgesic. I use magnesium sulfate at a dose of 1/2 c. to 50 gallons water to treat constipated fish.


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## plantbrain (Dec 15, 2003)

The Mg thing likely comes from the Reef folks and a couple of marine noxious algae, they elevate Mg to kill it off(Bryopsis).

No FW experiences...........the higher Ca, Mg etc might through salt stress, but most all FW algae are very very tolerant of salt stresses.

A good general targetting rule is that you will never need to dose more Mg, Ca, etc on down the line than you do N and K.

So if you have a max uptake of say 10ppm of N(not NO3), quite a bit, you will never need more than say 8-10ppm of Mg.

I think you will find that Mg will affect shrimp and plants before fish, some species might be sensitive, hard to say without real careful testing that is species specific.

Davis tap has 52ppm of Mg. No issues there.

Pretty high.........

Regards, 
Tom Barr


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## ER9 (Aug 2, 2008)

huh...interesting thanks for the insights.


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## plantbrain (Dec 15, 2003)

In other words, it's going to be very unlikely unless you go wild with the Epsom salt that anything bad will occur.............

Regards, 
Tom Barr


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