you know what, i've had the exact same problem. my softie's still a little small but getting bigger by the day, and stronger too. ever since he started eating more voraciously, his tank has been harder to keep clean. i don't understand it at all. especially since i use a huge turkey baster to remove his waste every now and then. (i don't catch all of it, i don't have tHaT much time on my hands). i've seen pictures on the internet of softies in perfectly clear water, and i am also interested in finding out how you can make this happen.
p.s. - i use a ten gallon tank with a topfin15 power filter. i replace the filter cartridge and do a complete tank clean-out with water and scrubbing every two weeks.