Water Sprite is the best floating plant, but it is "tender" and thus easily digestible by herbivore fish. I know Mbuna will eat soft plants as they are omnivore, but I am not sure about other rift lake species but other members can advise if you have the species. Water Lettuce (Pistia stratiotes) may survive better, depending upon the fish. Maybe Frogbit too.
Nitrates in the tank are controlled by water changes, keeping the filoter clean, substrate cleaning at water changes, live fast-growing plants, not overstocking/overfeeding. You have zero nitrate in your tap water so that is good. Cichlids are sensitive to nitrate we now know, and it (nitrate) should be kept as low as you can in the circumstances, but definitely below 20 ppm.
Nitrates in the tank are controlled by water changes, keeping the filoter clean, substrate cleaning at water changes, live fast-growing plants, not overstocking/overfeeding. You have zero nitrate in your tap water so that is good. Cichlids are sensitive to nitrate we now know, and it (nitrate) should be kept as low as you can in the circumstances, but definitely below 20 ppm.