Oxygen content varies with temperature, which affects the growth rate and maximum size of fish.
This is why your guppy remains tiny, although you feed it nutritious food twice a day. “If you want bigger guppies,” Pauly says, “keep them in water as cold as they can tolerate and make sure your tank is well aerated.”
Aeration, indeed, works wonders for fish in captivity, and this is why fish farmers aerate their ponds: not only does this prevent early morning mass mortalities (when naturally dissolved oxygen is at its lowest), but the fish convert their food much better than without aeration. Aquaculture practitioners have known this for at least half a century, but until Pauly incorporated this into his theory, biologists had not seen the link to fish growth.