I kept my RTBS in a 29g from the time I bought (about 2 years ago at .5") him until around a year ago. He grew fine in it but when I put him in a 55g, he took off like a weed and is now the second biggest fish in there after my adult female Bristlenose (roughly 4" with the RTBS just behind at 3.8"). Definitely don't do anything smaller than 30 gallons for a RTBS.