Hi and Welcome
"Rainbow Sharks have a very aggressive nature and are known to be very territorial."
Rainbow Shark Maximum Standard Length:
Fully-grown at 6"/15cm.
Minimum Tank Size:
"We wouldn't recommend keeping one of these in a tank smaller than 48" x 18" x 18"/120cm x 45cm x 45cm/255 litres."
heres the links:
http://www.seriouslyfish.com/profile.php?genus=Epalzeorhynchos&species=frenatum&id=234
http://e0.aqua-fish.net/show.php?h=rainbowshark
Your BN should be fine in that tank and will grow to about 12cm
ATM i am feeding them micro pellets by Hikari and am running out, im trying to get them on to flake foodbut they don't seem intereted enven when i crush it up?
What do you Feed your Ember tetras or other small tetras?