Hi Tommo
Following is a link showing mixing disasters with certain types of fish and other companions
http://www.caudata.org/cc/articles/Mixing_disasters.shtml#id26 .
You also have to be aware of the needs of the fish (as well as axolotl) - some tend to try and mix tropicals with axolotls (different temperature needs). Goldfish will nibble gills; fish also tend to excrete loads adding to bioload of tank. Some have used white clouds.
Have a search round the forum!
If you do intend using fish or any other axolotl companions to the tank (even as feeder fish); it is advised that you MUST quarantine them for a minimum of 30 days (I say minimum - in some cases quarantine may be required longer) to ensure the fish/axolotl is indeed healthy and doesn't carry any germs that could be passed onto your resident.
When quarantining feeder fish, if they are used - then it's better to try and breed your own feeder fish to ensure the offspring are from a healthy known source. This will cut down on the amount of problems people have from buying feeder fish from the petstore/breeder and just dumping them in their tanks to feed their axolotl only to discover their axie becomes sick/stressed for some reason (usually on introduction of possibly unquarantined sick fish).
Another thing to avoid doing - don't add any tankwater or filter media from the petshop/aquarium you buy the axolotl from (or for that matter fish if you buy them) to your tank. The water could come from a diseased tank, which you don't want mixing with your tankwater. Tip the axie into a plastic container with lid (or colander/sieve you don't intend using again) and then tip the axie into the tank, if setup, or tub of dechlorinated water.
Has your tank already been setup/cycled. See:
http://www.caudata.org/cc/articles/cyclingEDK.shtml
If not, a few things you should consider:
1. Don't use any product called "cycle" to try and cycle your tank. Despite what petshop assistants etc... state this doesn't work. It carries dead bacteria which will not speed the bacteria in your tank.
2. Buy some freshwater test kits for ammonia, nitrites and nitrates. The best sort are the test tube variety as they're more reliable. If you can afford to buy the freshwater master test kit sets. Most will have all three tests mentioned as well as extras like low or high pH test kits and/or GH/KH Test kits. The most important ones to test during cycling, and on a regular weekly basis (if tank is established/cycled) are ammonia, nitrites and nitrates. See the above link. If unable to afford the full test kit - then buy the ammonia one first then buy the nitrites next.
3. You don't need to buy products like pH up or down; or something to lock/bind ammonia or get rid of nitrates. Invariably this is a waste of money and don't fix whatever the underlying problem is (if any) in the tank. Frequent partial only waterchanges (20=30%) on a daily or every 2nd day basis (rather than weekly period) will lower toxins in the water (ie ammonia/nitrites) but still allow it to cycle.
4. Only product needed really is a water ager/conditioner or dechlorinater to remove chlorine and chloramines from tapwater.
5. Don't use gravel or undergravel filters - Gravel can be swallowed, even if handfeeding, and can cause impactions, leading in some cases to prolapse and eventual death (at worst). Axolotls will swallow almost anything smaller than the size of their mouth. Don't forget juvenile axolotls grow so by the time they reach adulthood a 2cm+ piece of pebble/gravel can/will be swallowed. Not a pretty sight when they're excreting it. Undergravel filters also harbour a lot of gunk beneath the filter plate which does not get cleaned unless you clean the gravel out completely and lift the plate. These can eventually cause water quality problems.
6. Invest in a turkey baster - great for spotcleaning.
7. If you don't have one get a gravel/siphon cleaner or even a cutoff piece of hose for siphoning water/waterchanging.
8. A thermometer
9. Clipon fans or pedestal fan (will be needed for the summer months).
NB I've moved this to Beginner's part of this forum.