55 Gallon Aquarium
1 Black Kuhli Loach
2 Black Skirt Tetras
4 Angelfish (3 Marble, 1 Gold)
1 Pleco (can't grow larger than 2 inches)
2 Glass Fish
2 Blue Sunset Gourami
3 African Dwarf Frog
1 Dieing Mollie
I wouldn't expect mollys to do well in this mixture. Mollys tend to thrive much better in tanks that have salt (marine salt preferrably) in the water. None of the other fish listed here can tolerate salt the way the mollys tend to prefer. If you'd like to try keeping mollys again, I would suggest in a tank of their own. 2 - 3 tablespoons per gallon is the basic dosage of salt for a molly tank. Please don't add this to your 55, your other fish and frogs will suffer dearly if you do.
Water test kits are something that should go home with every new fish tank, even before the fish. Without a test kit there is no way to monitor the cycling process, and also when something goes wrong, without a kit on hand, many times people find it's too late to help the fish by the time a reliable kit is found, purchased, understood, and used. I suggest monitoring test results in tanks every month or 2, even if they appear healthy. Things like ammonia, nitrite, nitrate, and pH can fluctuate very quickly, nitrates tend to build up over long periods of time, which, if not tested for on a regular basis can become quite toxic before noticed. Ammonia and nitrite are highly toxic even in trace amounts. All of these things are invisible. Things like high nitrate levels can also affect pH, causing it to drop quickly to levels that many fish can't survive.
How long has your tank been set up? The more we know about your tank and its history the easier it is to help you to determine what, if anything, is wrong.