The symtoms are sounding more like internal paracites. The problem here would be identification, without performing autopsy's on those that are gone and having the proper facilities to test, treatment would be hit and miss. If you are setting up a new tank, if possible, I would suggest running the tank fishless for 30 days, as most paracites would be gone from lack of a host.
If possible qt the remaining fish in seperate containers, syphoning out the bottom of the qt tanks daily. Many intestinal paracites can only be transmitted to others by being injusted. If you know someone that has a microscope and you can get some of the waste from the fish you may be able to find out if it is a intestinal paracite by finding eggs or larva in the waste. As far as chemicals to treat for internal paracites, the treatment will cause stress to the fish, so the wrong treatment will probably kill the fish.
The only other possible cause would be from intestinal impaction,which can be caused by feeding a restrictive diet, frozen foods not fully thawed.
These are just my opinions, and I hope that they help.
Rich