Our two have gone from your size to 26" in about 18 months.They eat prawn both in and out of shell,mussel,cockle,whitebait and trout and can easily hold their own at feeding time with our red tail who is about same size.They still eat pellets as a suplement and earthworms.Feed every day till about 18" then every other day till 24" then every 3-5 days after.I love all my fish but if I could only keep one type it would be them.If you need more info on net name has changed now to oxydoras niger.