What's the distance between the light fixture and your frogbit?
Back then it was about 5 inches from the water surface. I had it raised to about 10 inches thinking it was melting them. I didn't have a cover just an open top tank and I thought that, that might be the problem because of the temp might be fluctuating but I have seen other people without tops and they seem to do fine. I recently grabbed a cover for the top but my water lettuce and hornwort still is not thriving. With the cover on top the light sits right on top of it. Don't know if that would be a problem. I forgot to mention that the temp of the tank was about 83 degrees. I keep SA dwarf cichlids so they like the tank a little warmer but recently dropped the temp to about 78 degrees. As for the sponge filters I don't think they are the problem as I have the air line knotted so the surface movement is restricted a little. Apistos don't like a lot of water movement. Would it be worth it to add more iron or trace?