my suggestions:
if you don't want to buy a kH test kit, bring a sample of water to your LFS to test... IME, most LFS will do it.
from that kH reading and your pH levels, you can measure the CO2 levels... (there should be a chart at www.thekrib.com ). I think the danger zone is when the CO2 levels reaches around 35 ppm... but you should check the krib for the actual numbers.
also, I would start adding some of the flourish... but also some potassium nitrate to increase the nitrate level (to about 10-20ppm). But increase it slowly (over a few days) since a rapid increase can kill the fishes. The potassium wouldn't hurt either.. you can find potassium nitrate at a hydroponics shop...
if your nitrate levels are correct, then i really think the nitrates are your limiting factor. Meaning your plants aren't using up the phosphates fast enough.. and so the algae is feeding off of it.