I'm with Anna here, best to treat the whole tank. Ich spores live in the gravel once the spots go (2nd stage) and reinfect any fish they can get to. Also ich only breaks out coz of stress (overcrowding, bullies - that kind of thing) and/or bad water conditions. Bettas are extremely hardy fish and for Citrus to get ich, something's wrong in the tank. If you haven't got testers, take a little of the water from the tank to your lfs and ask them to test it for you (doesn't normally cost anything). Treat the tank, correct the water and I'm sure everything will be ok.