I've had the same "issue" too, wouldn't call it a problem, the bubbles never
smell... and I know I've got at least 2-3" of gravel.... and a UGF, but I
make the bubbles, by using a wooden skewer and aggitate the gravel to
release some "stuff" and every so often, if I take the skewer and catch a
part of the UGF, I can pick the UGF up a little, and bubbles come from
everywhere in that plate... I've smelled for the sulphur,but not present,
never smells, and is rarely funky... actually, it scared me once, and a
peacock came over and chased the bubble and inhaled it... spit it back
out.... I was totally expecting her to keel over.... but she didn't....

Could it be just oxygen, because I have some good growers....

It's not oxygen, and it's not toxic gas. It's plain old air. Air from the
airpump used to drive the UGF is getting caught under the filter plates, and
when there's enough of it, it bursts up through the gravel.

Ideally, you want another inch or so of gravel -- enough so that the air
doesn't pop up through the gravel. It's supposed to drive water circulation
through the gravel, and if the air is coming up through the plates instead of
the lift tubes, the UGF is not working as designed.




Leigh

http://www.fortunecity.com/lavender/halloween/881/