Join Date: Oct 2010
Location: Victoria, Australia
Goldfish grow incredibly fast. I believe in an ideal environment they are supposed to reach their full size in a couple of years.
When goldfish are housed in tanks that are too small for them, they can suffer from the effects of stunted growth.
Goldfish also produce an astonishing amount of ammonia for their size so even small fish require tanks with generous filtration.
It is good of you to take in your brother's goldfish, but based on your post it sounds like you may not be aware that they are quite a big responsibility. If your goldfish is a comet you are looking at a fish capable of growing up to and beyond a foot in length.
Goldfish also are capable of living for decades so it is a big commitment.