I have a 39 gallon tank with an external Aqueon filter, can't remember the size but the fish shop (reputable, not a petco or petsmart place!) states the filter is the right size. Set up for 2 months. 2 gouramis, 2 neon tetras, 1 zebra danio, 1 odessa barb, 1 albino catfish (not sure what kind, but it's like an inch long and we have been assured it won't be very big) and an albino chocolate pleco. There has been a lot of poop on the bottom of the tank since the addition of the pleco. I have done two 20% water changes with vacuuming the gravel in the last two weeks, and there's still more poop to be found. Do I need an under gravel filter, submersible filter, add some critters that clean up and leave less waste behind? I'd like to add a couple more fish but not until I figure this out. There is a tallish rock decoration on the side that the filter is on, should that be moved to the opposite side of the tank?