If you have a way to rehome it that is 100% foolproof, you could get away with it. While not ideal, 2-3 months in a 20 gallon would be ok. Just don't tell the tang police, and have a plan to get it out the moment you see it glass-surfing. With that said, tangs are not actually the best way of controlling algae. The best way to do that is to remove the factors behind the initial growth. Reducing the photoperiod to 7-8 hours daily, removing nutrients, reduced feeding, and manual removal will rapidly reduce the problem. You could also add a tuxedo urchin, which will leave a path of bare rock behind it as it moves. I once added an urchin to my 20 gallon to control bubble algae. While it ignored the bubbles, it vacuumed up sea lettuce, hair algae, and film algae. I hope that this helps!