Like piper said, an RTBS in anything less than a 30 gallon is a bad idea. they are active and territorial fish and grow to 6" so they do need their space.
I notice you have the following in your signature:
60L Tank with
- 1 x Betta (Crown Tail)
- 4 x Fancy Guppies
- 1 x Platy fry
- 3 x Otocinclus Affinis
- 3 x Zebra Loach
Is this the tank you were thinking of putting the RTBS in? Very bad idea. Actualy, having the betta with guppies in there is also quite risky in itself and the zebra loaches are known to occasionaly nip (not a good idea for a crowntail!) and would be better off in a large tank. If you do get another 15 gallon, consider seperating these fish into both and then just adding more to each tank so that your stocking is less likely to cause problems for you and your fish. An RTBS wouldn't work though regardless - it would compete with the loaches, probably attack the guppies and betta because of being crowded, and deffinately would stress everyone, including itself.