That must have been quite a surprise! Now, what to do? These need the temperature between 21 and 26C so if you can raise it just a tad and give him 5 pals of his own kind, your tank will be fine for keeping him (other tank mates permitting). If you can't raise the temperature or if other tank mates wouldn't suit a pack of zebra loaches then the best thing would be to try to track down where he came from and return him