1-- you added a whole lot of plants. Didn't you say you got 100 stems from an auction?
2-- You changed 40% of the water
3-- you turned off the filter
4-- you disrupted the substrate
5-- you left things in the dark after all this
Adding a bunch of plants suddenly will of course be disruptive to the system. Changing 40% of the water will lower the O2 since the water isn't well aerated when raw from the pipes. Plants consume oxygen and produce CO2 in the dark, and to exacerbate the situation, you turned the pumps off so they couldn't correct the gas saturation balance.
The disruption of the substrate was probably the least of the problem if it contributed at all.
Some sort of poisoning doesn't seem to be the case since only the gill-breathers were affected and everything is fine now, so I'd have to say that suffocation is the most likely suspect.
Doing any of the things you did would, on their own, not be a problem. However, you did them all at once, and that was just too much.
|