Well, now we covered way too many possibilities and variables -
-- parasites,
-- bacterial infection,
-- inadequate tanmates,
-- inadequate water parameters, and
-- a house plumbing/tap water change.
We need to start knocking some of these off otherwise we will be shooting in the dark til the fish die. The easiest one to rule out is water parameters. If you don't have a good test kit, you can take a sample of your water to an LFS - most test for free.
Without knowing the water parameters AND being sure of them, there is little sense in analyzing this situation further. This cannot be overstated.
If your house plumbing has been just changed and the uncured PVC glue is leaching into your tap water and other unforseen bad things too (detergents, lubricants, heavy metals like iron, nickel, cadmium, copper, etc.), more frequent water changes will not help (much), may even make it worse, unless you filter the tap water through a charcoal filter, like, e.g., Brita, etc. for drinking purposes... but then again, one needs to know what exactly been done to the plumbing system and if/how the tap water has been changed.
