I can't speak for axolotl's preference but I had an oscar who would spit out europeans because of some yellowish excretion. Currently raising africans, no yellow gunk and they do breed like mad at room temp but in my experience they're not as heat tolerant as suggested. 3 days in 90 degree weather did one bin in
. The one big advantage of the europeans was that they didn't wander around like the africans tend too- not crippling losses but dead worms all over the floor can be a pain.