Now I'm just guessing as I haven't seen any readings as of yet , were these the only 2 inhabitants?
before we get to far ahead get some readings you'd be surprised how much they can help.
so we stay on the right trail also check salinity reading , this could be another direction . too much salt not enough as you did a water change you mentioned , whats your water source does the water have heavy metals or even chlorine or other contaminants in it , is it RO/DI water