In a past life, I worked at a pet store, and one of the most heartbreaking things I saw on a daily basis was people who came in, little kid in tow, asking for a goldfish to put in a bowl. Goldfish are big, messy fish, and they grow to be 8-12 inches long. Depending on how persuasive I was, these folks went home with either a sad child or a sad fish.
The truth is that fishbowls and tiny tanks are not really suitable for any fish. Stores sell them because people buy them, and people buy them because they want something that will sit on their table and look cute. But if you want to keep fish, you owe it to them to research what they actually need, and assemble a tank that takes care of those needs.
Make it at least 20 Liters
There are fish that can thrive in a small tank. Not many! But a few, like bettas, can do well here. The catch is that we’re not talking about a vase or a mason jar; fishkeeping experts agree that the minimum size to properly care for a betta is about a five-gallon (20 L) tank.
That size gives you enough room for a bunch of live or silk plants (plastic plants will shred their fins) and other features that will give the fish plenty of hiding places. You’ll also need a heater to keep the tank at an appropriate temperature (room temp is too cold for bettas) and either a filter or a strict schedule of water changes. Here’s a care sheet for bettas.
If you want more than one fish, an 20 L tank could be a suitable home for a few guppies. Here’s a list from r/Aquariums of easy-to-care-for beginner species, including tetras, small catfish, shrimps, and snails. They won’t all be happy in a five-gallon, though, especially the ones that need to live in groups. If you want more than one type of fish, you should probably just go ahead and get a 76 L tank, which is still “small” in the aquarium world.
No, they don’t “grow to the size of their tank”
Many fish you see in stores are babies, and they will get bigger over the years. (To be honest, I think there’s an ethical problem here with the stores that market bala sharks and plecos to people who don’t already have enormous aquariums at home, but that’s for another story.) Definitely look up the adult size of any fish you want to buy, and expect it to reach that size.
One way people justify keeping fish in small containers, even after they learn how big they will get, is by saying they heard that fish “only grow to the size of their tank,” and therefore any size tank they put the fish in is fine. If you’ve heard this, I’m truly sorry to tell you that you have been lied to.
Fish sometimes are smaller in captivity than in the wild if their growth is stunted due to poor heath or malnourishment, or if they simply die before reaching their adult size. That’s the grain of truth in this myth. If a big fish is healthy and well cared for, it will get big.
Yes, you need to feed them
This is another myth that exists to make people feel better about too-small bowls, and that is detrimental to the fish. Fish and plants do not make up a “complete ecosystem” in which each feeds the other. If you put a betta in a vase with a plant, you still need to feed the betta.
Not feeding your fish (or under-feeding them) will keep the tank or bowl a little cleaner, but you’re not giving your pet the food they need to stay healthy. Please feed your fish. While you’re at it, don’t put bettas in vases; they need access to the surface of their water, and they need more room than a vase provides.
Filter it properly
Small tanks and bowls allow the fish’s waste to build up, which can poison them. (Would you want to swim all day in your own pee?) Water changes can help, but most aquariums should have a filter. The question of what kind you need, and how to set it up, is more complicated than we can summarize here; you’ll want to do your research on what your chosen species needs. (This guide is a good start.)
What to do with that fishbowl
All of this advice boils down to “put your fish in a real aquarium.” So what should you do if you just want something small and cute on your desk? Consider repurposing the fishbowl for a tiny planted aquascape.
Wabi-kusa are little balls of substrate with tiny plants growing out of them; you can place them in a small glass container like a fishbowl, so they’re like an aquatic houseplant. Marimo, or moss balls, are another unusual aquatic plant that can be kept in a tank or vase.
If you’d like to take the idea of an aquatic houseplant a bit farther, look up “nano” aquaria, which are usually filled with tiny plants and occasionally (if conditions are right) small creatures like dwarf shrimp. So you can have a cute little aquatic habitat in a small space. It just isn’t going to look anything like a lonely goldfish in a too-small bowl.