Nikki Casali wrote:

I have four female mollies in my planted tank. Three are white, the
other is mottled black. One white molly is so buoyant at the moment that
her dorsal fin is permanently out of water. She tries to swim down but
is thwarted by the buoyancy. Another white molly seems to have the same
problem, but is less severe. This molly keeps taking air from the
surface and then dives down, pauses, then realises she wants another
gulp and then she's straight back up. The gill activity looks normal on
all. The black molly is behaving normally. All the other fish are
behaving normally, including SAE, angels, otocinclus etc.

The CO2 level could possibly be as high as 30ppm. The plants are
pearling loads. What could be causing the mollies to constantly gulp for
air and getting stuck with too much inside their swimming bladder?

All the mollies have now resumed normal behaviour after their bout of
"wind". I'm starting to think that this behaviour is not too abnormal.
I've read that fish sometimes swallow air to aid in digestion. I fed
them dry flakes earlier so this could have been the trigger.

Nikki