Best way to diffuse CO2 into Aquarium?

Folks I think I'm having a problem getting CO2 to diffuse into my tank.
My well water right out of the tap is very soft and acidic: about a pH of
6.2, a KH of 80ppm and a GH of 75. (Perfect for Discus, which I keep.) I
have a small CO2 tank connected with silicon tubing. I put the outlet tube
into the inlet of my Rena Filstar canister filter. I give the 55 gallon
tank about 12 hours of light with two 65 watt compact flourcent bulbs. But
I'm still troubled by 'black-green' algae and minimal plant growth. I keep
the water at 85F. I fertilize regularly with Trace and Flourish and keep the
Nitrate level at 20ppm.

I've tried all different types of plants and while they survive and
grow a bit, there is no growth such as I see in pictures of other hobbyist's
tanks. Any advise or suggestions on how to get the CO2 to stay in the water
or what I could be doing wrong is appreciated. From the specifications
above above I'd think the plants should be growing out of the tank! Thanks
for any help.

JC