I found a relatively inexpensive micron filter in response to my need to get rid of green algae.
Initially, I purchased a 295GPH powerhead and a Hagen Quick Filter attachment, which I had read here worked well to filter algae. Unfortunately, the filter was no where near fine enough to remove the small algae, so I went looking for another type. I found one at Lowes, of all places!
Standard Capacity Replacement Filter
It is a 2 micron filter, and is sold in two packs for $10. It fits directly onto the power head, and all I had to do was plug the other end. I used a large cork.
The filter is really fine, and lowers the flow of the powerhead by probably 80% or more, but in 2 days, my 90G tank was crystal clear! I still have another unused filter for the next time, but I think that with some reverse water pressure, I can clean the used filter.
Total cost was $25 for the powerhead and $10 for the filters.