well, essentually the "dirty water" is just natural bacteria colonies...i mean, a standard UGF
uses a pump that distributes the same amount of water that an average cannister filter would.
essentially the "dirty" stuff under the rocks is left over food and poop...eventually that waste is broken down, but, it takes time. thats why if a pump on a UGF
goes bad in a small tank there can be VERY bad results.
a cannister does the same thing any filter would do, UGF
,over-the-back, or otherwise. it provides a harbor for benificial bacteria....filter floss or UGF
's catching debri is just a side affect.
at any rate, i dont see why it would be a bad thing, i dont see how it would serve any extra good than having a cannister and a seperate UGF
...I personally like to run multiple filters incase one of them goes bad, this is especially true with UGF
's. technically, if the filters were run seperately, if either one of the pumps went bad there is a lesser chance of tank occupants dieing.
i do know one thing, if you are running a smaller tank, i wouldnt get a UGF
tray covering the whole tank. i guess its cause i am overly careful. i just would worry about what would happen if the power went out or a pump went bad on a UGF
in a sub 20 gallon tank.