jimipot,
People have used a lot of different plastic materials for bacteria to grow on in a wet/dry.
Most have large units and want to save a few bucks by not purchasing the bioballs.
I can't vouch for any of these material choices, and don't know if they are more effective or not. I've never tried them.
I've heard of people on here using everything from plastic hair curlers to little toy plastic army men.
This link should take you to a Q&A discussion on just this very topic, although you may notice that this information is somewhat outdated, I'm sure the bacteria don't care.
Me - I use live rock, deep sandbed and still use some bioballs on my small FOWLR tank like Mr. Salty, but on my reef tank - no wet/dry or bioballs.
Later ...
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