Well, I have an LDA33 Snowall in with two BN's, mine dont go for prawns tbh, they do like the sinking general botto feeder pellets tho and hit the courgettes within seconds of delivery.
My male is 6" and the female about 3" both were the same size and bought together on same day just under a year ago. They both are quite unsociable now, but the courgette brings them out, funny tho both used to love cucumber but no longer go near it, but they do now go for romaine lettuce which they never used too.