Disclaimer: this is NOT my fish. I help to take care of it (water changes), but it's not mine. It belongs to my university project team.

It's a comet goldfish, as far as I can tell. When my teammates purchased it, they got ~10 "feeder fish". I'm not going to go into all of the details of what this fish went through, but it is the only one alive.

Its body is ~3 inches long, and it's been that length for almost a year now. It's living in a 29 gallon tank; yes, I know that's not big enough. If/when it gets bigger, I will work on finding a more suitable aquarium.

Here's the thing-- this fish has been in my team's possession for almost 2 years now, and has been in the larger tank for almost a year and I haven't seen any growth in the past year.

Has his growth already been stunted? Will he ever grow to full size? If so, how long does it usually take?

that seems a bit weird, I have two comets that the kids won at the fair last June and they have each grown about 2 inchs since then, I know they can get 10-12" if they have the room to grow, if hes by himself in a 29 gallon he should be growing more. Is he being fed regularly?

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90 gallon, 1 Iridescent shark, 2 Pleco, 3 Goldfish, 5 Cory cats, 2 Nerite snails
5 gallon hex, Blue Crowntail Betta, 1 nerite snail,

Yes, although not every single day. More like 4-5 times a week, because people don't remember to feed daily-- they're all wrapped up in the project.

This fish is a survivor. He lived in a cooler (!) without food for quite a while before somebody found him, and then he ended up in a tiny 1.5 gallon (at least it had a filter) for 6 months. I suppose it's entirely possible that this stunted his growth.

his growth may already be stunted, although i had some feeder comets from walmart a few years ago and eventually moved them to my mom's smallish pond, they have never grown over about 4 inches when they have about 500 gallons of water to themselves so maybe just bad genetics in feeders?

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"Being ignorant is not so much a Shame, as being unwilling to learn".-- Benjamin Franklin

I agree, feeders arent bred from the best of stock to begin with then they are crammed in little tanks,

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90 gallon, 1 Iridescent shark, 2 Pleco, 3 Goldfish, 5 Cory cats, 2 Nerite snails
5 gallon hex, Blue Crowntail Betta, 1 nerite snail,

Okay. I'll see what happens. He'll stay in the 29 gallon for now, but if he grows a lot bigger, I'll try to get my team to rehome him.

Is there anything I can keep in there with him? Some sort of small fish or snail? (Again, if he grows, I'll move him out)

I have had comets and the like for years..bought as tiny feeders and now 8 years later most of them or 9" or a tad smaller. They live in my pond outside for the spring/summer/fall and are brought inside my large stock tank for the winter. In my experience the tank size for this fish is not stunting its growth for the moment, but moreso the lack of feeding. Goldfish don't have a stomach, that's why keepers consider them "dirty" because they should eat more frequently than other fish. Eat more-poop more-dirty fish. Give this dude more frequent feedings and he/she will thrive. (oh and a larger tank when you can as well)!

Okay, I'll work on getting him fed more often. The problem is that college students are less than reliable... But I'll try.

Most fish do a good portion of their growing in the 1st year or two, then continue to grow but much more slowly for the rest of their lives. A fish kept under bad conditions for it's first year is unlikely ever to reach it's full size potential. It will probably still grow a little though. Very stunted fish are usually easy to spot because their eyes and head looks too big for the body. Of course kind of like people or dogs even under the best conditions they won't all grow to the same size. Most fancy goldfish breeds have been bred for shape not size, they actually tend not to be so hardy or so large as the simpler forms of goldfish like commets. The growth of feeder goldfish probably has more to do with the conditions they are kept in early in life than genetics.

Feeding every other day is fine for a goldfish. It is actually good for their digestion to skip feeding once a week. Less food may lead to slower growth but that isn't a big problem as long as it's not looking skinny. It's tummy should look nicely rounded, not sunken. Some live food and veg added to the diet might speed up growth but these foods are very messy so should only be given if compensated for by extra water changes and a good filter or it might do more harm than good.

I think the 29 gallon is a fine size for one goldfish, unless it gets very large. Good filteration and regular water changes are important. If your filter is not good enough ammonia and nitrites will not be processed fast enough and they are very bad for fish. Even with a good filter nitrates will build up with time. In low amounts nitrates are not a big problem but in larger amounts stunt fish growth and cause stress and disease. Water changes keep nitrates down. Live plants also consume nitrates so you can try using live plants but goldfish often eat them!