Deciding the right aquarium size can be a quite confusing decision to make. We’re going to go over the most popular aquarium sizes so that you can decide what will be the best fish tank size for you. 

How to Find How many Gallons in your Fish Tank?

Typically, buying an aquarium at a pet store will tell you the number of gallons in that tank. But in case you don`t know the exact size of the aquarium, this is How to figure out how many gallons are in your fish tank. 

All you need are a measuring tape or any measuring device and a calculator. So we’re going to measure the length, width, and height of the fish tank and multiply them together.

Fish tank size Calculation Formula
Fish tank size Calculation Formula

As an example, assume we’ve got a fish tank with a length of 24 inches, a width of 12 inches, and a height of 20 inches. So the number of gallons will be the length times width times the height. 

Now you have the tank capacity in cubic inches. By dividing this number by 231, we can get the tank capacity with gallons. Because the volume of one gallon of water is 231 cubic inches.

These are the standard aquarium sizes in the hobby. Many aquarium suppliers like Aqueon manufactures good quality aquariums in standard sizes. In addition, there can be custom-made aquariums too.

Standard Aquarium Sizes
Standard Aquarium Sizes

5 & 10 Gallons Fish Tanks (Small Aquariums)

Five or ten-gallon aquariums are great for fish keepers who prefer tiny nano fish, where the fish stay small and don’t need tons of space. 

The first pro of these aquariums is the size. You can place them anywhere, on your desk, bookshelf, or even on your nightstand. 

10 Gallons Aquarium
10 Gallons Aquarium

As well as, they are easy to move. You can drain half of the water, and it will only weigh 30 or 40 pounds. So you can just carry it wherever you want. So It will be a good solution if you’re somebody who is always moving stuff around. 

Also, they’re super cheap to set up. Their starter kits cost less than 75$ and include everything you need. Also, you could go bare bones and just filter it with a sponge filter and save even more money. 

But unfortunately, not all is perfect with these small aquariums. There are some disadvantages too. 

First, you’re limited to the types of fish that you can keep in them. With 5-gallon fish tanks being so small, you’re not able to keep goldfish, cichlids, or some of your larger community fish. But you’re good to go if you’re into the small fish or shrimp.  

Also, as they are being so small; they can get dirty really fast. So will need to do frequent maintenance and water changes. But such a tiny tank will not take you like 10 minutes to clean it.

20, 29 & 40 Gallons Fish Tanks (Mid Sized Aquariums)

With a 20-gallon fish tank, your options are similar to those of 5 or 10 gallons aquariums. But you will get a serious amount of weight. A 20-gallon aquarium weighs around 225 pounds when it’s full of water.

29 Gallons Fish tank
29 Gallons Aquarium

So you should pay attention to your fish tank stand and where you place the aquarium. Hence you don’t need your floor to crash, make sure it’s in a good secure spot in your home. It doesn’t need to be on a concrete surface like for bigger tanks. But you’ve still got to pay attention to where you put it. 

20 to 29-gallon fish tanks are the things that start to get serious. You’ve got more options to choose from as far as your filters, lights, and many other things. 

With 20-gallon aquariums, you can still use sponge filters efficiently. But with this size tank, it’s time to start thinking about power filters too. Because you may want more options like being able to adjust the flow rate and add things like bio media. 

Also, you can start getting into some larger fish. But let’s not go crazy. You’re still not ready for discus or goldfish. But many of the community fish, livebearers, and even some of your dwarf cichlids will be perfect in this size tank. 

You can add fish like Angels in there with a few tetras, and you’re gonna have an amazing tank with a lot of activity. They’ll not only thrive but also have enough room to breed, which is a great way to take this hobby up a level. 

40-gallon breeder tanks might be only 11 gallons more than 29 gallons aquariums. But considering the footprint, it’s a huge step up. The extra six inches in length and depth gives you all kinds of opportunities for some amazing aquascapes and breeding projects.

Filtering a 40-gallon fish tank is again going to be open to your preference. Sponge filters are acceptable, and power filters are even better. 

55 & 75-gallon Fish Tanks (Mid Sized Aquariums)

When you select a 55-gallon or 75-gallon aquarium, everything is bigger. Usually, a  55-gallon aquarium weighs over 600 pounds when it is full of water. So It’s better to keep this type of aquarium on a solid surface like a basement floor. 

Once you get to 55 gallons and up, you should consider getting a power filtration system like hanging on the backs or smaller canister filters. 

Also, with a 55-gallon aquarium, you’re opening the door to much bigger fish. A lot of South and Central American Cichlids, African cichlids, and even some catfish will be perfectly fine in this size tank. Oscars, blood parrots, jack Dempsey, Sevrums, and the list goes on. 

But just make sure to do your research on the type of cichlids you plan to keep because you need to ensure that they’re not going to get over 12 inches long.

125 & 150 Gallon Fish Tanks (Large Aquariums)

125 and 150 gallon tanks can keep a wide range of aquarium fish in the hobby. Most freshwater fish will be OK with this tank sizes except the extra large fish like Arapima or Red-tailed catfish.

Also, you can put 4 to 5 Oscars,10 to 12 Discus, or you could go crazy and put 150 Rummy nose Tetras in there.

Oscar Fish tank
Oscar Fish tank

For a 125 gallons tank, you need some giant filtration. You can get a couple of large hang-on-the-back filters or a big canister filter.

Fish tanks in this range will weigh around 1500 pounds with water, So be careful about the tank’s placement.

Compared to 125 gallons, 150 gallons tank have the same length and depth. But you get an extra 7 inches of height, which makes a huge difference. 

125 Gallons Fish tank
125 Gallons Aquarium

What’s the best Fish Tank size for beginners? 

Many people who are brand new to aquarium hobby might automatically assume that larger aquariums are more difficult to keep than smaller ones. But this is absolutely not true. 

Assume you have three fish in a tiny aquarium, it doesn’t take long for the water to get all fouled up from uneaten food and fish waste. But putting those same fish in a larger tank will take much longer for all that water to get nasty. 

So larger the tank means easier to keep, and you won’t have to do nearly as much work on it as you would a small one. It does not mean that new fish keepers should start with only big tanks. But it’s better to get the biggest possible tank with your budget.

Also, you should decide what fish you want to keep before buying your tank. Because then only you’ll know the exact size of aquarium you need.


I am Prasanjaya who is an aquarium hobbyist for nearly a decade. I wish to help fishkeepers to get accurate and helpful information regarding every aspect of fishkeeping.

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