An Undergravel filter's primary purpose is to be a biological filter. Biological filtration is the most important filtration function performed on our aquariums. On a 20 long, the typical U/G plate has four ports for the clear riser tubes to be inserted into. I would suggest that the two inner ports have lift tubes with either an air stone in each port or a small power head (like your 550) at the top of each lift tube. Disableing your undergravel with out replacing it with a completly easablished (mature) biological filter will most certainly cause water quality issues. The suggestion of replacing your U/G with a hang on box filter is risky at best. THe hang on type filters are great at what their primary function is, Trapping dirt and chemical filtration. Neither of these function efficently process waste and although the water looks great, it often is unstable. Consider this, if un eatten fish food is laying on the bottom of your aquarium rotting, clearly is can and most likly will cause water quality problems. If un eatten fish food is sucked into your hang on filter and is trapped in the filter pad or sponge, it is still in your water, rotting and fouling you water. The ideal set up on most aquariums will include Biological filtration (in you case the undergravel) to process waste to maintain 0 PPM ammonia & nitrite. and good mechanical / chemical filtetration such as the aquaclear mention by another post. Mechanical filtration simply means some type of efficient dirt trap and chemical filtration in this case means the use of activated carbon to remove dissolved solids that may stain the water or give it an odor. I'm sure you'll hear someone stating that undergravel filter are worthless, etc... but when set up how they are intended to be used and maintained properly (vacumm the gravel often) they worl very well at a very small price. Try adding a good power filter such as the aqua clear or penguin and get the u/g running properly and your aquarium should do great for you....Joel
