I would go with bioceramic rings or balls, not plastic bioballs, and definitely not lava rock. Bioballs do provide biological filtration but not enough in such a small space, lava rock works great at firstbut becomes impacted and slowly loses bio-availability (aka surface area for bacteria to grow because all the pores get impacted with crud and there is no way to clean it all out).
"[Wabi-sabi] nurtures all that is authentic by acknowledging three simple realities: nothing lasts, nothing is finished, and nothing is perfect." -original author unknown-
125g marine reef tank, 6 gallon fluval edge minimalist aquascape (in progress), 1 gallon aqueon evo 2 betta tank.