Wanted: Help With Floating Plants

Hi all, just joined and looking for advice on floating plants. I purchased four types of floating plants salvinia cuccullata, salvinia oblongifolia, hygroryza aristata, and frogbit. I have limited experience with submersed plants and I thought this would be fairly easy with floating plants. When I received the plants they were all healthy and as a fail safe I put some of them in an outdoor tub. These get about 6 hours of sunlight and are in just rain water, no fertilizer and are doing well. My plant tank is a different story. For lighting I have 24 inch coralife HO, 2 bulb. I have tried different bulbs and the current ones are Geismann super flora. I have tried different water from straight ro to a mix of tap and aquarium water. Tank is 15 gallon 12 x 12 x 24. I keep the light about 12 inches from the top because of heat. I have tried a couple different Flourish products. The plants are not doing well, even the frogbit which I thought would be the easiest. The cuccullata cupped leaves turn brown from inside the bottom and this brown slowly turns the whole leaf brown. The leaves of the oblongifolia also turn brown. The plants keep growing, but the older growth keeps turning brown. If you were to start this plant set up, how would you go about doing it?