I suspect your water stats, adding 7 fish to a tank of your size that previously housed 2 fish is quite a strain on the biological filter.
It would also help if we could ID your fish better and know how many of each kind you had/have in your tank.
Was your zebra fish a
zebra danio or was it a bottom feeder, perhaps a loach similar to
the zebra loach? Your neon fish sounds a bit like a
neon tetra. Your sucking fish could have been any of a number of things (and you may have been sold one of the larger species as a dwarf species, it happens frequently). Can you give us some idea on the size?
Otos are probably the smallest, there are also many many species of
pleco.
Chinese algae eaters are commonly sold and also come in a golden color with the same body shape, as are
Siamese algae eaters.
All of the fish I've listed except for the oto and the neon tetra are too large or active for a tank of your size I'm afraid. Mollys need not only a larger tank but also brackish water or very very clean fresh water to thrive.
I'd strongly suggest getting a good freshwater master test kit (API makes a good one) that covers pH, ammonia, nitrites and nitrates. In small tanks, small changes can become big problems very quickly, so water testing is even more essential. If you take water to your local fish store to be tested, make sure that you have them write down numbers for each of these things for you, often they'll just tell you that your water is "fine" even when it isn't.
