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I have no idea what I did wrong…

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Saturday morning is weekly maintenance.

3 tanks and everybody is happy and content.


1 tank, disaster…

and I have no clue what I did wrong.

my 20 High glofish tank had 7 of 8 glofish die, 6 of 6 green neon tetras die, 2 of 6 pygmy cories die.  My 6 emerald green cories are not too active but none have died, at least not yet….


siphoned a few gallons of water, wiped front and side glass, trimmed and replanted some plants, proceeded to do 50% water change.  I am darn sure I added Fritz complete, but my Co op test strips say my tap water has no  discernible chlorine.

I may have forgotten to add Equilibrium and alkaline buffer that I normally do.  I normally keep both at 5-6 degrees hardness. I would have sworn Ihad added it as normal, but….

Dosed easy green, easy iron to bring back up.

cleaned top glass panel as usual and went to do 4th tank.


came back to test nitrates as I usually do after ferts circulate for a while.  Dead fish floating in the current…


I immediately did a deep water change assuming I massively overdosed something.  Fished out the dead bodies….


did a full water test before adding anything….Ammonia 1 ppm, normal, my tap water is chronically 1 ppm ammonia, 0 nitrites, ph 7.2, nitrates 5 ppm, Gh 0, KH 0..  


dosed Equilibrium and Alkaline buffer to GH 6, KH6.  Dosed Easy Green to 20 ppm. Nitrates…

My tap GH, KH are normally 1-2 degrees…  

Given an initial 50% water change even if I forgot to dose, I have a hard time believing a reduction of 3 degrees hardness would cause death on the scale I experienced as quickly as I did.  Maybe half an hour.

given co op test strips not detecting discernible chlorine I sort of doubt forgetting dechlorinator would kill the fish in half an hour…

3 airstones in tank so I sort of doubt if I went heavy on the dechlor it would cause this issue…


any guesses?   I am at a loss for an explanation….

no change in behaviour in any of the other tanks…


I am stumped…



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My water is very soft like yours (GH 0) and I too have had fish die right after a large-ish water change.  (I keep the tank GH around 6-8 by adding wonder shells ie its quite different from the tap water).  I have concluded that some fish may be quite sensitive to sudden changes in the concentration of calcium and other minerals in the water.  I now try to stick to 25% or less water changes, more frequently if necessary.

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As a rule I am mixing up Equilibrium and Alkalinity buffer as I am refilling and add it as it is filling to restore hardness to prefill level.


possibly I omitted that step, but realistically it would at most be a drop from 6 degrees to 3.  


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