There's no need to lower the water for axolotls to swim to gulp. They have no problem doing so when they need to. An airstone can be added to the tank if needbe.
Axolotls do float in hangout modes but excessive floating, especially during the summer months (if you don't have a cooler system on your axie tank - as most don't) can mean it is stressed and there is a temperature/water quality problem. Heat and Waste will build up in lowered water (or a small tank) faster than in a fully filled or 3/4 filled tank, in particular, if waterchanges are irregular (fortnightly). The warmer the tankwater the less oxygen there is.