[Avalon]

Are you adding any fertilizers? It sounds as if you aren't, or you're not using the correct ones anyway. There is a way around it: The sunlight will be fine, but I would reduce your water changes to once every two weeks and possibly longer (maybe once per month). I suspect the sun is driving plant growth quickly enough that the plants are being deprived of nitrogen and/or phosphate. You can allow them to build by reducing your water change frequency; your plants will use the built up nutrients for food. You case of algae sounds like a small one, so it should be addressed with small changes regardless of what you choose to do.