I'm sure you wouldn't let it happen - well a responsable pet owner wouldn't. But my sister-in-law has 4 adult axi's in an outside pond, it has no covering and she lets the dogs runthrough it and play in it, it's under a tree and all the gumleavges fall in it. The water is soooooo green and dirty, she has no airation or filtration (a waterfall would provide adiquate airation) there is only one small plant and a single log for them to hide in/under. Every so offten she drops a couple of pelets into the pond - but not enough food for all 4 adults.
AND SHE WONDERS WHY THEY WON'T BREED!!!!
There isn't anything I can do really, except not give her any of my babies.
If you put them in a pond - make sure it is covered, because anything could get them like, birds, cats, dogs etc....
If you put fish in there, put somthing small like guppies - and these will be a good food source. However just be careful, as said in a previous post, if the fish are too big or an agressive type, they will nibble on the axi's feathery bits.
I can't make the final decission for you, but just think about it before you do anything, and dont be affraid to ask any questions.
GOOD LUCK - what ever you do!