I have 2 retics (tea cups) which I keep with discus and large rasbora's in a 6x2x2(feet) they are about 7inches at the moment and doing fine in this sized tank.
I previously had a 15inch diameter retic which would have been ok but the tail was at least 15inches again giving a body lenght of about 30inches hence the reason I gave it to a friend with a larger tank.
