I have tried the following so far in the tank and this is how they went down.
1 - ledge - somewhere good to sleep - good to hide under.
2 - Plants - somewhere good to sleep / hide in / does my bum look big in this?
3 - other axies - good to nudge and sit on.
4. fresh water clams - good to nudge.
5. moss balls - snore.
6. lass jars - food.....fooooood.... wait no food ..... bored now. double check

nope no food.
7. upside down plant saucer - home sweet home
8. shrimp - I can hunt it - chase it - and if I am quick enough ..... no missed. (can cause fights b carefull with multiple axies)
9. bubbles - like a summer breeze/spar treatment)
10. ping pong ball - WHAT IS THAT!!! (RUN AND HIDE)
11. Pet treat ball - MMMMMMMM... I can nudge it AND I get food
12. mesh fish breeder - Can hide under it hang off side ..poke it.... POKE.... POKE!!!! are there shrimps in there!!!!!
Yes my axies are fat

try them with everything see what they like the most...if they can sleep on it nudge it or eat it they will like what you give them.