It sounds like your tank is way overstocked. Likely your biofiltration is not coping. As stated above, test your water by a liquid test tube kit (not test sticks!) and post numbers. I do not expect your parameters to be fine.
But in the unlikely event they are "fine", it can be grossly misleading too because there is too little water to instantly dilute toxins, most prominently nitrite. Look up "brown blood disease".
There also could be shortage of dissolved oxygen.
But in the unlikely event they are "fine", it can be grossly misleading too because there is too little water to instantly dilute toxins, most prominently nitrite. Look up "brown blood disease".
There also could be shortage of dissolved oxygen.