Well, if they're not producing enough oxygen, it's certainly not because my tank isn't well oxygenated. It's a 20-gallon-high, and I have an AC50 hob with the flow control valve open full throttle, plus a four-inch airstone that bubbles vigorously. Plenty of current and should be plenty of oxygen. None of my fish are at the surface struggling to breathe, so it must be okay. Thus far, my mossballs are still a rich, deep green, and I haven't seen any browning or turning white, at all. They've been in my tank for several months.
-- bigmamafish