The air coming out of fish tanks has a strange smellI saw a fish tank which has a machine that keeps emitting bubbles. What is its purpose and what...

I am not sure about the smell that comes out of thee fish tanks.
The smell could be that of the fishes or other material such as sand, stones, and some vegetation
that we may add to the water in the tank for improving its look. The smell could also be caused
by stagnant water.

However, the nature of bubbles is very clear.
These are air bubbles. The air is circulated in the fish tank to keep the water in the tank
supplied with enough oxygen for the fishes to breathe. In natural environment there is sufficient
quantity of oxygen dissolves in the water. This is because there is usually a huge mass of water
and the oxygen in it does no get depleted that fast by absorption by the fishes. Further the
oxygen in natural water gets replenished by the natural vegetation in the water. In rivers and
streams the water itself gets replenished. This does not happen in small quantity of stagnant
water in fish tanks. Therefore, it becomes necessary to circulate some air in the tank so that
some oxygen in the air gets dissolved in the water for the fishes to be able to get enough oxygen
from the water.