I have finished cycling a 48ltr tank which we bough for my son as he wants a fancy fantail. Its been cycling it for 7 weeks and its all complete now

Can anyone advise me what fish, if any, could be out in with a fantail please? The ones we've seen are very small (i know they dont stay that way) But it HAS to be a fantail im afraid...
I know that i can only house one in the tank as they grow large and have high waste, but i was thinking of maybe adding a snail? When i mentioned this at my aquatics store they said that i could also add 5 diano's but i think thats too many...am i right?
Any advice would be appreciated as we've gone to all the bother of cycling a tank properly and dont want to mess it up now.
Thanks