Maybe I didnt get you right – you say you recently did a move ?
But you also say they are in the same tank for 2 years ?
If its the same tank – for two years, I would like to ask, – how often do you ‘clean’ the tank ? Do you just do water changes ?
Reason I am asking, is that in the low ph, acidic tanks, – if theres a lot of waste – which builds up, at some point you can get an amonium peak situation. But of course, if you run a filter on it, – this is less likely, but it could be.
That could give the kind of symptoms you describe …