It depends on the species of fish and what your trying to achieve. There are some species of fish which are commonly used as feeders, guppies, white cloud mountain minnows etc. They should be quarantined or bred yourself in an isolated system to reduce the chances of introducing disease. Most will disappear quite fast with a few of the smarter ones hanging about and eventually may breed but they generally get wiped out so you need to replenish them occasionally. Fish should only be used as a food source rather than tank mates, there is absolutely no benefit to an axolotl to share a tank with fish other than as snacks. Some species of fish like plecos will suck slime of axolotls bodies and may eventually kill them , larger fish may attack axolotl gills damaging them and allowing fungal infections to take hold, other fish have spikes or spines which can injure and even kill. People need to use some common sense when choosing to add fish to an axolotl tank, some are ok and some are potentially lethal. There are people who mix axolotls with bigger fish and report no problems but why bother and take that risk ?