just out of curiosity:
could one inject O2 into a tank (CO2, non C02 or unplanted) in order to increase the fish load?

wish i had the money, space, time etc. to try it but.....

whats O2 cost any how

I would think that oxygen content would not be the limiting factor with fish load. Being that oxygen content can be kept nearly at equilibrium with the atmosphere just by having very good circulation. Plants would just add that much more o2. More important would be a lack of sufficient biological filtration, and the need for water changes being so frequent, that it would be impractical. Besides the needs of the individual fish for space.