Bio balls will do well for "filtration" in terms of nitrification, but you will need some form of nitrate removal or frequent water changes as well. As you can see in my pics, I put large balls of algae in the tank. There are other ways, but this was easier.
Conventional wisdom is that anemones only do well in established tanks, but have numerous 10 gallon tanks and a 20 gallon that I set up for this purpose on the day of introduction with a small live rock and Arag-alive substrate and they are all doing well. In theory, the bio balls should be all you need though, so long as the tank is well cycled and stable. BRS made a video about this and suggested a marine pure block because anemones are easy to remove from it, but they seem over priced to me for what they are. You should get the same effect with bio balls. I haven't tried either though. You will also need good flow and lighting of course. I use an Aquaclear 50 HOB and it work great. Put a sponge filter on the intake, or your nem will get sucked into it.
Lighting:
If you don't have corals, I have found that planted tank lights work great and are cheaper so long as you have good PAR. The nems will look better under a reef light, but white light may provide better growth (the jury is still out on this). Macro algae respond better to the white light for sure
I just had my black widow split today BTW. its the first picture. That tank is one month old. The 20 gallon long with 6 RBTAs is a year old
Lominie s20 reef light
Nicrew SkyLED Plus. (technically fresh water, but has nice blue mode, however you may need full white as well like shown to get adequate PAR)
Some random top fin LED supplementing natural day light
Lominie s20 reef light and Aqua knight V2 (has nice ramp timer function, best choice for a 12g cube IMO)
