I have the same problem here every year... The water temp starts to warm up, and the string algae takes off like crazy, then it takes nearly a month before the temperatures stabilize enough that the water plants really start sucking up the nutrients. Sure my moneywort and some other plants are starting to grow, but there's just not enough yet to effectively clean the water.
I wonder how something like tulips would do in the water? That's a very early starting plant, so I wonder if they would be suitable for cleaning the water at the beginning of Spring?