It is not the fish's size that would worry me about the tank although it will not like being so restricted. The aggressiveness of the fish will make it much more active and the more cramped an aggressive fish feels, the worse the aggression gets. He is likely to become so upset he (or she) would run into the things perhaps even the side of the tank and injure himself/herself. You would not be able to have any other fish even if you did try this and there is no gaurantee that this would work even with keeping this fish alone.
It is up to you whether you want to try to do this but be prepared with a back-up plan like even returning it to the store where it was purchased. You will probably not get anything for it, but they may take it back because of its aggression especially if you or your brother have been good customers. Sometimes they will give a store credit.
I hope you can work something out. It may be that the fish will settle down in the tank, but if you are removing it due to aggression, I do not see it being better.
I do want to welcome you to Fishlore.com. It has been my pleasure to post to you. Please feel free to browse the site, question anything you may wish to and comment as well. We have many members from all age groups, all nationalities, and many different interests.
Rose
