When you walk into a pet store and see those tiny, colorful fish swimming peacefully in their tanks, it’s easy to imagine bringing them home to live happily ever after in your 10-gallon setup. But here’s the thing: many of those fish won’t stay small for long — and some grow way bigger than most people expect!
If you’re just getting started with fishkeeping or thinking about adding a new species to your tank, it’s super important to know how big your new fish friend will get when fully grown. Let’s take a look at some freshwater and saltwater fish that may surprise you with their full-grown size.
Freshwater Fish That Get Big
1. Oscar Fish
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Starts out: Around 2–3 inches in pet stores.
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Grows to: 10–12 inches in captivity, up to 18 inches or more in the wild!
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Oscars are smart, interactive, and a bit feisty. But they need space and can be aggressive, especially in cramped tanks. A solo Oscar should have at least a 55-gallon tank.
2. Common Plecos (Plecostomus)
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Starts out: Just a few inches long.
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Grows to: A whopping 18–24 inches!
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These algae eaters are often bought as “tank cleaners,” but the common pleco is anything but small. They’re poop machines too, so get ready for serious filtration. Want a smaller version? Try a Bristlenose or Clown Pleco instead.
3. Angelfish
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Starts out: Cute and tiny.
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Grows to: Up to 10 inches tall (including fins).
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These elegant fish need tall tanks and calm tankmates. They’re semi-aggressive when breeding and enjoy swimming space.
4. Goldfish
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Starts out: Everyone’s first pet fish, right?
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Grows to: 8–12 inches or more, depending on the variety.
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Goldfish are not bowl-sized. With good care, they live 10+ years and get surprisingly large. Forget the bowl — they need tanks of 30+ gallons each!
5. Bala Sharks
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Starts out: Around 3 inches.
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Grows to: Up to 14 inches.
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These peaceful but very active fish are schooling types — they need at least three to five buddies to feel safe, which means a 100+ gallon tank is a must.
Saltwater Fish That Need Space
1. Groupers
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Starts out: Often sold at 2–3 inches.
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Grows to: Over 3 feet long depending on the species
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Groupers are beautiful but better suited for public aquariums or massive home tanks. They’re aggressive eaters and need tons of space as well as large tank makes to avoid making them into snacks.
2. Tangs (like Dory, a Regal Tang)
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Starts out: Tiny and cute.
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Grows to: 8–15 inches depending on species
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Blue/Regal Tangs, Yellow Tangs, Sohal Tangs, Clown Tangs and their many cousins are fast swimmers who love open space. They are often seen in the wild in massive schools and need oxygen-rich water and get aggressive if cramped.
3. Marine Angelfish
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Starts out: Flashy and small.
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Grows to: Up to 15 inches for species like the Emperor Angelfish, all the way to 24 inches for the Grey Angelfish.
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They’re territorial and graze constantly, so give them room to roam and lots of rockwork. They will feed on corals so they are not generally recommended for reef tanks.
Why Size Matters
When a fish doesn’t have enough space:
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It can become stressed or sick, potentially causing your other fish to get sick as well.
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It may stop growing properly (called stunting).
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It might become aggressive — even to previously peaceful tankmates.
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You may end up having to rehome it or upgrade your tank unexpectedly.
Always plan for the adult size, not the size you see in the store!
Be a Smart Fishkeeper
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Do your research before buying.
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Ask questions!
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Start with the right tank size — or be ready to upgrade as your fish grow.
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Think long-term — fish can live for years, even decades!
Final Thoughts
It’s easy to fall in love with a tiny fish at the store, but giving them the right space helps them stay healthy and happy. Whether you’re a beginner or experienced aquarist, planning ahead will save you time, money, and fishy heartache.
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