is a snail
will have come in on your plants, can leave it or can get rid of it, up to you. it won't harm your fish, it may nibble plants, problem with snails is while 1/2 may be cute you can v quickly get overrun with them which is then a nightmare to sort!
personally i would get rid of it (he should survive in a pond if you have one nearby you can be humane and take him there) rather than risk being overrun later