fish hiding

my molly has spent the past two days hiding in a small castle... he will come out to eat but soon goes back into hiding. No new fish have been introduced, and the existing fish seem to be co-existing well (i haven't noticed the molly being chased or nipped at) After just having lost 3 fish last week, I'm worried. What could be the problem?

Also in the tank are:

-dalmatian molly
-2 white tetras
-2 black tetras
-cory
-pleco
(the fish in question is a balloon molly)

all these fish have been together for 2 months now
last week we lost a swordtail, a gold dust molly, and a bala shark

did a 30% water change on saturday
pH is 7.0
ammonia is 0
nitrates unknown (i need to get a new test kit)
salt at about 2 tsp/gallon and temp at 84 (i'm slowly coming down after an ich treatment)

obviously, the tank has been under a lot of stress the past few weeks. I don't know what else to do!