The Pea Green is driving me crazy G

My pond is 3 years old, and goes through this every year. I am using "pond
blocks" but they are not working. I have a Tetra 2.5 gallon filter (pond is
about 650 gallons). I am working on convering a 50 gallon preform into a
bio-filter. Hope that will help. The pond get a medium amount of light,
some parts of the pond are in the sun all day.

Plants are starting to come up, and I have a little hornswort and anacranis
(sp) in the upper 100 gallon area.

Will try to get some pics up on my website soon.

Ideas, suggestions???

Thanks
Marc