If it's toxic gas, you should be able to tell by the smell. It will stink
to
high heaven.

It could be oxygen. If plants are growing well, they can pearl under the
gravel, as well as from their leaves. But usually the bubbles aren't that
big.
And, aside from the chain swords, you don't have the kind of rooted
plants
that do this.

Do you have a UGF? If so, then it sounds like what happens when you don't
have
enough gravel over it.

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....