Box Filter set-up?

[oliviaprismo]

I just bought a box filter, and I was going to layer it with a sponge and gravel, but I'm lost as to what else might be beneficial to my axolotl? I was just wondering if anyone else used box filters, and what you've found worked well?

 

[Urmat]

Perhaps you could throw some bio balls in there? I used to keep those in the canister filter I had.