Put the water sprite in a breeding net and let it grow, when it gets too big you can put some in the tank for the fish to eat.
If the fish are eating the plants, offer them algae flakes or pellets, and try them on pumpkin, zucchini, peas, carrot. Make sure they are free of chemicals. You can either cook them or feed them raw, most fish like them partially cooked so they are softer.
You can grow Duckweed and Water Sprite outdoors in plastic tubs, and when you have plenty, move some into the tank for the fish to eat.