I find these responses interesting because I've never had an issue with my frogbit in poorer lighting, or with extra nutrients (in fact, they seemed to enjoy the latter)...whenever I had decay, it would always start on the outer edges of the leaf and begin to "melt"--turns out is because of moisture getting on the top of the leaves. Unlike duckweed, which does just fine underwater, frogbit leaves are designed to stay strictly ON TOP of the water, with small amounts of moisture being shed by the waxy coating. I would suggest trimming off all dead/decaying parts (those seemed to drag the rest of the healthy plant under the water), and making sure that your HOB filter isn't splashing too much of a mist onto the leaves. Even a sponge filter was able to disrupt mine; you're definitely doing the right thing by using a ring to keep them in one area!