james wrote:
I started with an impeller/reservoir/charcoal filter together with an
undergravel filter in a 55 gallon tank. I recently switched to a Fluval
304, but when I did, I turned off the undergravel filter, and almost
killed my fish because it seems they were starving for O2. I turned on
the UG and dropped an airstone in. I didn't consider that the fluval
filter was a completely closed system and doesn't aerate the water.

Fish are doing ok now but I don't think I've improved my tank much,
since I'm still running an UG filter, and I still have an annoyingly
loud pump. I do like the fluval, but I really want to get rid of this
pump. (Can you recommend an air pump that's truly quiet?) (Why do I
have to import O2 anyway?)

Oxygen absorption in the water is a matter of water surface area and
surface movement. Your UGF has bubbles leaving it, which agitate the
surface ... that is how the oxygen is replenished in the water (not the
bubbles themselves, but the bubbles bursting and moving the surface).

Since I have never used one, I don't know whether the return on the
Fluval can be made to agitate the surface of the water ... you might try
that if it is possible.

ANOTHER factor in your fishes' reaction could be the following scenario:
If you installed the Fluval and turned off the UGF at essentially the
same time. Why? Because your biological filter was all in your gravel
and not at all in the Fluval at first. It would take several days for
the bacteria to populate the Fluval ... and even then you might want to
turn _down_ your UGF for several days in a row before turning it off
completely. Non- or under-functioning biological filter means rising
ammonia and/or nitrite levels, both of which are fatal to fish.

Finally, many people use a submersible power head on UGF instead of
airstones. Power heads are quiet, more efficient than airstones at
moving water through the UGF ... hence even better biological filter.
And the output of the power head can be set to agititate the surface for
good oxygen exchange.

Hope this helps,
Dennis