Eclipse 25gal tank, green cloudy water?

Hey folks,

I've got an Eclipse 25 gallon tank with five Black Tetra's, seven
Danios, and common Plecostomus.

Temperature is kept at ~75F, filter cartridge changes are made on
schedule (about every two weeks, just when water starts spilling over
the overflow spillway), tank is cycled and has a well-populated
bio-wheel. Partial water changes are done every few weeks (2 weeks to
one month, 15-25% each time).

When I was cycling the tank with the Danios, a substantial amount of
algae built up on the glass. After the tank was cycled, I moved the
Tetras and Pleco from my old 10 gallon tank to the Eclipse, and within a
day or two the Pleco had eaten all the algae and the tank was clean and
clear.

This lasted about a month or two after the new fish were transferred
over. Now, there's not really any algae deposits on the walls (a little
under the heater where the Pleco can't get to), but there's some really
ethereal "wispy" green stuff along the gravel at the bottom (no
undergravel filter, though I vacuum it with the siphon when doing water
changes) along with some solid fish-waste.

Additionally, the water has a distinctive greenish tint and appears
fairly cloudy. It's extremely cloudy in the morning just as I turn on
the light, and it gets slightly less cloudy during the day but never
really clears up.

Its not terribly attractive to look at, and I can't imagine that it's
very pleasant for the fish either.

Any speculations as to what might be a cause of this and what could be
done to correct this?

Cheers!

--
Pete Stephenson
HeyPete.com