If he continue to get worst and not improve, then I would consider treatment. I cannot tell you when, because I don't see him day to day. The main thing if if he is getting better or not.
Equipment for treatment is minimal (maybe I think it is minimal because I have so much aquarium stuff around) but if you don't have them, you will need a few days to gather. Antibiotic is cheap but you need somebody to prescribe it for you. If the anemone is too far gone and smelly (bacterial cause necrosis, rotting of the inner organs)
I doubt that any treatment would be effective
antibiotic
20 g tank
PH
clip on light
heater
air pump and air stone