Another thing, most live breeders like mollies and platys do like some salt. As a matter of fact, when acclimated properly, mollies can be in SW
We have a pair of silver lyretail mollies we bought at Petsmart that started to act the same way you're describing. Once I started adding marine salt to the tank (just started adding the salt last weekend, it's a brackish set up) they did much better. Now that they are at a specific gravity of 1.004 they are happy as can be.
I would suggest adding a small amount of aquarium salt or marine/reef salt. The corys can withstand some, not much tho. The platys and mollies will love it. Do it slowly tho. Mix in a clean cup (no soap residue) some water from the tank (after you have tested it yourself and can be sure it's safe for the fish) with a 1/4 teaspoon of the salt. Let it desolve then slowly pour it in a little at a time.