bubbles ?

[onlyme]

I have a 2x2x1 (95 litres) tank that I used as grow out for 38 kribs. I put them in as 1.5cm fish and now they are approaching full grown. Healthy growing fish, java moss exploded that I had to keep removing some and later I added an emergent plant in a pot which also thrived. It was filtered by a single air driven sponge filter. I had no fish losses even though I did no maintainance, even water changes in a 3 month period. The number 2 crushed marble substated was so dirty I was worried about the 2 common bristle noses so I decided to vaccum the bottom and generally clean the tank up and add some more filtration. As I was distrubing the gravel with the suction hose the gravel was releasing bubbles. As there was no smell and the tank was healthy I doubt it was hydrogen sulphide and the gravel is only 2~2.5 cm thick. Could it be that under these conditions the nitrogen cycle was completing and the bubbles were N2 ? If not, any ideas what the gas was ?

Craig.