well, to start, open brain corals are an LPS coral. YOu'll want to make sure the PH, Alk, and CA levels in your tank are appropriate for LPS. This will help ensure it's growth. Lighting can vary from specimen to specimen with these. If it's going in your 20g H you'll need at the bare minimum, PC lighting. Normal output tubes may not cut it. Also, you'll want to target feed occasionally, I attempted to feed mine at least once a week, twice is good in some instances. Beware of shrimps and similiar greedy inverts, they will steal food from the brain while he's trying to eat.

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Other than that, care is similar to most corals, ensure good water quality and make sure the flow is good but not enough to rip the tissue from the bone. Target feed at night, since that's when the feeders will most likely be out, but in all honesty, he'll probably learn to feed during the daytime or once food is added to the tank, he'll "smell" it or whatever corals do to know food is near and you'll see tenticles when he's trying to feed.
Hope this helps, if you have any specific questions feel free to hollar.