It sounds like you have a moderate light tank with some supplemental CO2. You also seem to have a moderate fish load. You say soft water, do you have dKH? Are you using a CO2 drop checker? You say no ferts, I am wondering why not. Are the plants growing well? With the light and CO2 that you have, the plants are probably hungry for nutrients. The algae can grow in extreme conditions, but not so much the plants. Also you probably need a larger plant mass, particularly of rapid growth type. Anyway, that is how I would look at it.