Everything You Need to Know Before Buying a 20 Gallon Tank

Are you thinking of getting a fish tank for your home or office? It can be a great addition to any space.  Before you go for it, there are a few things you need to consider. 

In this article, you’ll learn how big a 20 gallon aquarium is, how much it weighs, and what equipment you’ll need to keep your new fish happy.

If you’re a beginner fish keeper, please check out our introduction guide Beginners’ guide to choosing the best fish tank. As you are getting your new aquarium set up, please see our guide Where to put an aquarium for helpful tips.

20 gallon aquarium dimensions
A 20 gallon aquarium is the perfect size for your home or office. We have one in both places !

How big is it? 

As you plan for your fish tank, you should start with the tank dimensions. This tank size is the most popular one around as it’s small enough to fit most every home yet big enough to offer room for your fish and aquatic plants. We started with a 20 gallon fish tank before moving on to larger tanks.  In fact, we still have that first one. We moved it to the waiting room of our veterinary clinic for our pet parents and pets to enjoy. Please note we are discussing a freshwater aquarium in this article. Salt water aquariums use similar tanks, filters, pumps etc. but have much higher maintenance issues. We enjoy the ease of maintenance of fresh water even though we love the beauty of salt water fish.

There are 2 main types of 20 gallon fish tanks. Both are a rectangular fish tank. The standard size and the “long” version. Regular ones are usually 24″ long by 12″ wide by 16″ high. A 20 gallon long fish tank is 30″ long by 12″ wide by 12″ high. The long fish tank isn’t as deep, but is wider. This makes the long fish tank appear bigger while giving more space for fish and plants to spread out. That said, you still want to be careful about the quantity of fish you put in. Choosing a non aggressive fish species like the Neon Tetra will help too. like Both a “tall or high” aquarium and a long one weigh about the same. With water, gravel (substrate), and equipment, you are looking at about 225-250 pounds. 

A premium version is known as a rimless aquarium. Most tanks have metal frames on the top and bottom. These provide additional strength to hold the glass panels in place.  A rimless type has no braces which gives a more open look.  They cost more as the manufacturer has to take more steps to ensure the glass panels stay together with all the pressure of the water wanting to push the glass panels apart.

What size heater should you get?

A heater keeps the water temperature consistent. Fish in general, don’t like large swings in temperature. In colder climates, it’s a must have to keep your fish and plants healthy and happy. The formula we like is for every gallon of water, you’ll need 5 watts of heater to warm the water up 10 degrees above the room its in. We suggest you have a hood to keep the heat in and reduce evaporation. This also helps keep your finned friends safe from jumping out. Or in our case, our family of curious cats. So for a 20 gallon aquarium, a heater of 100 watts could keep the fish tank at 80 degrees if the room it’s in is 70 degrees. Most freshwater fish like temperatures around 78-80 degrees. Some tropical fish from warmer climates like water temperatures above 85 degrees. Remember that placement of your tank can have an impact on your heater needs. For example, if it’s near a window that gets sun all day, your heater will work less since the sun warms the water during the day. On the other hand, a tank in a basement where with no windows might run more often. See our in depth guide on How to heat up and cool down an aquarium.

What type of light should I get?

Aquarium fish illuminated with a light from the top of the tank
Lighting will make the colors of your fish really pop while keeping your aquatic plants healthy.

If you have a regular 20 gallon aquarium, a 24″ long LED light would work as that’s how long the aquarium tank is. If you have a long fish tank that is 30″ wide, a 30″ LED light would work well. A hood with an integrated LED light is the perfect solution. We like LED lights as most let you adjust the light output and how white (Kelvin rating) the light is. This is important as your fish and aquarium plants will be happier with light that mimics their natural habitat. For more info, please read our post Do fish need light?

How much gravel should I have?

Gravel or substrate is helpful for fish to know where the bottom is. Otherwise the reflection from the bottom of the fish tank can confuse your fish. Having gravel on the bottom also looks nice. A good rule of thumb is 2 inches of substrate will work if you just have fish. This lets beneficial (good) bacteria grow. If it’s too deep, you risk having too little oxygen get down where excess fish food and waste goes. If left unattended, they can rot and cause issues for everything in your tank. If you also have live plants, they need a layer of nutrient rich substrate of about 1-2 inches underneath the 2 inch upper layer. And like all things you add to your aquarium, please wash it according to the directions on the package to remove any residues that may harm your fish or plants.

Aquarium stand considerations

This size of fish tank is small enough that you can place it on a sturdy shelf (remember, it can weigh close to 250 pounds !) or a sturdy desk. The key here is “sturdy”. The tank itself is heavy because it’s made of glass and each gallon of water weighs 8.34 pounds or just under 167 pounds for 20 gallons. A dedicated aquarium stand is ideal where you can make your aquarium match the decor in your home. There are metal stands, wood cabinets that have storage underneath (great for hiding equipment). The other consideration is to place your stand where you have an electrical outlet.  For extra protection, you could have your licensed electrician install a GFCI (Ground Fault Circuit Interrupter). The purpose of these is to detect when an electrical issue occurs and disconnects the power.  For example, if your electrical devices (hood, pump, etc.) contact something they shouldn’t (like the water or you), instead of shocking you or electrifying the water in your tank, it cuts off the power to the outlet.  You’ll see these safety devices in your kitchen and bathroom, and outdoors.  

The tall version is 6″ wider and the same width as the more common size. Remember this when planning your space requirements. You’ll also want to have a space around the tank instead of having a stand that is the exact dimensions as the footprint of your tank.  We learned this the hard way.  It’s very convenient to have that space to place things on when you are maintaining your tank or just feeding them.

Aquarium hood considerations

We recommend a hood to help with temperature control and to keep your finned friends in the tank. They also help to reduce evaporation. Another side benefit is they can contain a bit of the noise from your filter and air pump. We recommend a hood with a built in LED light.  If you have plants, we recommend looking for LED lights that are “full spectrum”.  This means the LED’s will provide the light spectrum that your aquatic plants need to flourish.  As I’m a bit lazy, I love having a light with a timer.  I program it to light up the aquarium every day so my plants get what they need. The dimensions of the aquarium hood for a regular tank is 24″ by 12″.  For the long aquarium, you’ll need a hood that is 30″ by 12″ so make sure you buy the correct size. 

There are different styles of hoods. If you have cats like we do, then having a hood which completely covers the top will be very helpful.  The best models have a hinged opening so you can feed and do maintenance. 

Filter considerations

Why do you need a filter? To maintain water quality. The fact is, your fish may not eat all the food you give before it settles on the bottom. Worse, after your fish eat all that food, they excrete wastes in the same water they live in. Yep, they poop and pee where they eat and live. All the excess fish food and fish waste sit on the bottom of your tank where it rots (decompose). As these things break down, they produce ammonia which is toxic to your fish and plants. This is where a filter comes in. Along with a pump to move the water through the filter, the materials inside the filter provide an environment for beneficial (good) bacteria to grow. The bacteria consume the ammonia and make nitrite. This isn’t good for your fish either. Thankfully, other bacteria eats the nitrite and releases nitrate which is way less toxic to your fish.

This is why you need proper filtration. But what about the different kinds? There are different types of filters that offer different maintenance requirements. In tank power filters have the filter media and pump all in the tank. The benefit of this is the water will reduce the sound made by the pump. The downside is when you have to do your regular maintenance on it, you have to pull everything out of the water and it can be a bit messy.  Canister filters have a canister which stores the filter media and are separate from the pump. Media filters also known as a gravel filter are screens placed under the gravel media. Over time, beneficial bacteria builds up in the media and helps remove the wastes that are sucked into the screen which is connected to the water pump.

The other part of the filter is the pump. It’s a simple device that moves the water around through the tank and through the filter. Most power filter designs also introduce oxygen into the water by cascading the water back into the tank. One thing to watch is some pumps may be a problem for small fish. The inlet pipe where the pump takes in the water from the tank may be large enough for smaller fish to be pulled in (remember the Disney movie Finding Nemo?). A quick way to remedy this is to place a piece of screen material that has holes large enough to let the water flow, but not let fish get sucked into the pump. It also protects your pump from larger debris (like a part of your plants) from getting stuck in the impellers (the blades that spin to move the water). 

How big of a pump do you need? Pumps are rated in GPH or Gallons Per Hour. We recommend using a 4 to 1 rule which is 4 times the size of your tank. This means a 20 gallon fish tank would need at least an 80 Gallon Per Hour filter. The pump has to move that water volume to ensure consistently clean water. We created an indepth look on aquarium filters. Please read our post How to choose the best filter for your aquarium.


As avid fish tank aficionados, we have found many different items work well to keep our aquariums running well. This means our fish and plants are healthy. For our in depth guide, please see our post on Stuff that you will need to maintain an aquarium. If you are just taking up the aquarium hobby, a 20 gallon aquarium kit is a great way to get started. In most cases, you have everything you need except water and fish. Many even include books about aquarium keeping. The beauty and joy your new tank will bring will make everyone in your household happy. Especially your cat. Kidding. But not really. Ask our cats. They love our aquariums but aren’t fans of the hoods as they keep them out. Thank goodness for fish tank hoods !

You’ve learned how big a 20 gallon aquarium is, how much it weighs, and what equipment you’ll need to keep your new fish happy.  With this knowledge, you can ensure your pet fish will be healthy and happy. And keeping your Pet Happy is what we all want. 


Question: How do I choose the right size tank for my fish?

Answer: The size of your fish tank will depend on the number and size of your fish. As a general rule, you should have at least 1 gallon of water per inch of fish, but larger fish will require more space. Consider the adult size of your fish when choosing a tank.

Question: What are the dimensions of a 20 gallon fish tank?

Answer: 24″ Long x 12″ Wide x 16″ High. A 20 gallon Tall aquarium is 30″ Long x 12″ Wide x 12″ High.

Question: How many fish can I keep in a 20 gallon tank?

Answer: The number of fish you can keep in a 20 gallon tank depends on their size and species. As a general rule, you should have no more than 1 inch of fish per gallon of water. For a 20 gallon tank, this means you could keep approximately 10-15 small fish, or 2-3 larger fish.

Question: Can I keep a Betta fish in a 20 gallon tank?

Answer: Yes, you can keep a Betta fish in a 20 gallon tank. However, they prefer shallow water, so make sure to provide plenty of hiding places and plants.

Question: Do I need a filter for my 20 gallon fish tank?

Answer: Yes, a filter is necessary for a 20 gallon fish tank. Filters help to remove waste and debris from the water which can be harmful to your fish if left untreated.

Question: How often should I clean my 20 gallon fish tank?

Answer: You should clean your 20 gallon fish tank every 2 to 3 weeks. This involves removing approximately 25% of the water and replacing it with fresh, treated water. You should also scrub the sides of the tank and any decorations or accessories to remove any algae or debris.

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