As much as you want to believe your tank is safe since you haven't had a problem in a year.. that tank is a death trap! Gold fish will eat your axies gills they don't have a stomach so they constantly eat and poop! I wouldn't call you filter a filter at all especially with all you have in that tank. carbon doesn't get rid of ammonia or anything all carbon does is make the water clear. Usually there is a little sponge in front of your filter and that's the "Biofilter" when all it really is is junk. Those rocks are definately small enough to be eaten.
Just because they are hand fed doesn't mean they wont suck up from time to time and get a rock. Also fish can transmit parasites and diseases. So I would.... get rid of the fish, get rid of the gravel and get sand, get some bio media in your filter like
this and if you need to put them back in soon get enough of
this to cycle your tank immediately and make sure the axolotls are in cold pure clean water to let them heal.