A 60L (~15 gallons) is really a bit small for an axolotl. The minimum is 75L (20gallons). That said, with proper filtration, you may be able to get away with it until they're full grown. Sponge filters are an excellent secondary filter. They offer good biological and mechanical filtration and are fairly inexpensive. I use a combination of sponge filters and canister filter on all of my tanks. My 75 gallon (283L) runs with a Fluval 407 canister and 2 large sponge filters. My 36G (136L) runs with a Fluval 307 canister and one sponge filter, and so on. A smaller tank would be fine with a HOB power filter and a sponge filter for added biological filtration. Sponge filters dont need to be replaced all that often (maybe 2-3 times a year). Just a good rinse off during water changes is plenty. Just make sure to rinse it in a bucket of dirty tank water and not tap water or you'll kill any good bacteria on the sponge.