I hate to tell you, but I think your tank is pretty much "done". You're probably near the max bioload and it looks like your plants are filling in nicely.
If you want to do something, the oto could probably use a friend or two. You might have to find a home for the one of the other schools though. I'm not sure what your filter can handle as I think you have a typo in listing it as "1000L/m". That's less than most fire hoses.