Yeah, just for the sake of it, test and post your findings. Ammonia, Nitrite, Nitrate, ph
, temp and salinity. I'm not sure why your fish are getting sick or behaving erratically but we have to start somewhere. Also, when you did the water change did you make sure the salinity, ph
and temps were the same? Large changes in these numbers will effect your fish. These may seem like no brainer things but I needed to ask.