That's a Melanotaenia fluviatilis or duboulayi. I would look up pictures of both and ID your rainbowfish that way. Either way if it is doing well, feeding, swimming around, etc. it will do fine alone. If in the future you upgrade to a 55 gallon or larger I would recommend adding in 5 more (with 3 males and 3 females) for the most vibrant coloration and activity.