[BTTRFLYGRL]

Have you checked your salinity? You are adding saltwater to replace evaporated water ...The water evaporates, not the salt..Salinity rises with evaporation. Top offs should be done with plain water-no salt added..You are not doing water changes by replacing evaporated water

You say this all started when you added a Clown...What kind of Clown? Was it tank raised or wild caught? This now raises the question of Brooklynella..Yes, other types fish can get Brook.
Your fish are probably hiding because they are sick..You need to get a qt set up and get them out. You need to get a proper diagnosis because Velvet and Brook are treated with different meds..It is possible you could give the fish 3 formalin dips-3 days apart and also treat them in a quarantine with copper if you are not sure what you are dealing with. You need to leave your tank fallow for at least 6 weeks to make sure that the parasite will die off