What are the water parameters?
- calcium / alkalinity
- general hardware
Did you add new fish, remove filter pad and either replace with a new one or wash it out in untreated/conditioned/not declorinated tap water.
How often usually are your water changes and how much do you change?
I would start with a number of wTer changes 15-20% for the first few, then 25% for a couple more in one day.
You can do the first 2 one and then the other. Then a few hours do the next 2.
It's done this qay so the water can gradually match closer to the tap water, slowly as to make the process/ change more gentle on a sick strwssed fish.
Make sure the water temp is matched up between tank and new water.
And that your dechlorinator works for chlorine And chloramine which water treatment facilities often use in summer time since chloramine is much stronger/longer lasting.