Have you tried using a brine shrimp net (the white very fine mesh nets... such a fine mesh infact that it looks like a solid piece of cloth) to skim the top? Also, if there is a substrate in the tank are you stirring it before or during the waterchange to get all the debris?
Do you also realize that by the time your clown loaches and pleco get to full size the tank would be stocked with just them (even 2-3 clown loaches and 1 common pleco would stock a 75g at adult size.) So the bioload will continue to get heavier as all the fish age, which will, probably, cause more floating debris.
