Last night I did my regular maintenance of all my tanks checked NH3 NO2 and NO3.
all tanks read 0 NO2 and less than 10 NO3 and one tank read about .25 NH3.
I topped up all my tanks and water changed the one with the ammonia.
This morning I got up and turned on my aquarium lights to find a dead male peacock in my 80 gallon (this tank did not get a water change just topped it up) and in my oto tank there was 1 dead female congo 7 dead cochu's and a dead oto ( again this was topped up and treated with Aqua plus my normal conditioner but not other changes). Tested the water for both today still reads 0 . What could cause such a massive fish death over night?
The only possibility with the cochus, congo and otto is that by filling the tank it put more water against the heater and caused a slight temperature spike. This wouldn't have changed more then 5 degrees though.
All fish were swimming fine last night/