Fish experts' help needed please...My fish are dying :(?

Sounds like your levels are off somewhere, prob. ammonia or nitrates, and stressing the fish. Get your water parameters checked (ammonia, nitrite, nitrate, ph). If you don't want to buy a test kit, take it to a pet store and have it tested. You're best bet would be to take it to a store specializing in fish if you have one available and they may be able to give you good advice about whats going on in there and how to fix it. 20 mollies would require 40 gallons at a minimum, so if your tank is smaller than that, you're probably seeing the effects of ammonia poisoning or high nitrates, especially only doing maintenance every other month. Nitrates are the least dangerous thing in your tank, but high levels are very dangerous and the only way to lower them is water changes. Switch over to 15% weekly water changes and a 25% once a month when you clean your filters, ornaments, and glass. Also try cutting back to feeding once a day and only what they can completely eat in around one minute (scoop out any extra after that). Like I said, get your water tested and post the levels on here. We could help you a lot more if we had that information.