I recently got 3 albino tiger barbs from a petco, four days ago. They were fine for the first day or two but then I noticed one of them has a sunken belly. Today I noticed that the second one has developed a sunken belly. I have them in a 5.5 gallon quarantine and I added filter media to the filter from a tank that I've never had problems with. I added salt to the tank before I got the fish but nothing else. since I've had no luck with finding info I thought I'd ask here as to what it could possibly be? They eat find but have gotten less active