Here is some info on the toxic gases. Google it.
They are not really air, they are pockets of poisonous gas. Fine sand often gets compacted together tightly enough that water does not circulate through the sand, so the sand becomes an environment that is very low in oxygen. Then, when organic matter (fish waste, food, plants, dead animals) is trapped in the sand, it is broken down by anerobic bacteria, which, as waste products, produce some toxic substances, especially hydrogen sulfide. All this has been known to occur in some gravel substrates, but it is much more likely in a sand substrate. These pockets of poison gas will eventually escape from the sand and, if there is a large enough amount of the gas, can poison the fish. The gas also smells really bad (something like sewage or rotten eggs).
You can prevent the buildup of this gas by making sure the sand isn't too deep (not more than a couple of inches deep), and by stirring the sand frequently. Sometimes the animals in the tank will do the stirring for you. I love MTS, they burrow constantly. stir up substrate pretty good if you have enough of them. they do however breed exceedingly slow for snails
