Yeah, barnacles require a relatively large amount of feed (primarily zooplankton for adult barnacles, as I understand it) that is hard to get to them in our aquariums. That said, if you're wanting to keep live barnacles, I'd suggest seeding the tank with pods and rotifers, dosing the tank with phyto regularly (specifically diatoms like Chaetoceros gracilis or Skeletonema costatum), and target feeding the barnacles with pods and/or Artemia nauplii preferably after they have been gut-loaded on the phyto (again, I'd specifically recommend using C. gracilis or S. costatum, or at least species from the same the same genus as these).
I know some barnacle species will spawn on a BBS (Baby Brine Shrimp - A.K.A. Artemia nauplii) diet if they're getting enough BBS (50 nauplii per barnacle per day was the lowest amount I've seen tested), but I don't know if this holds true for all barnacles species, so your mileage may vary with it. Similarly, I know that the larvae of a few different barnacle species have been reared successfully using the diatoms mentioned above (with the best success rates being found when the diatoms are fed in concentrations of 6x10^5 cells per ml), but other diatoms/phyto species have been shown to be ineffective/insufficient nutritionally for the larvae. So, that's why I'd recommend those ones specifically.
