uh, *gulp* I only wanted aquarium advice, woo..yikes! Wrong place?

Ok, so how do you do this? Stats first? OK:
20 gal undergravel filter and a Aqua Clear 150
22 fish and maybe snails as the 2 clowns just
love to dig a hole, rock by rock to eat something?
Really nice bright salmon colored ones, and their
fins are brillant and the black is dark black.

mm, lets see, if 1" per gallon, I am over stocked to
25" (?) ok, wait I must need a breakdown,

11 neons'
3 heaDLIGHTS, oops
3 black skirt tetras'
2 albino catfish (who insist onlaying eggs)
1 1"opihi pleco (i love its dancing algae feasting)
1 4" clown & 1 3" clown

Yikes! my question only was this is a 3 yr old tank and
I added some fert for my plants and am plaqued with the
hairy algae! Hints please? I have this Algae killer but its
old and doesn't seem to help