20 Gallon Fish Tank Review

The 20 gallon fish tank is one of the most popular fish tanks on our website. Not many people realize this but there are a few different 20 gallon fish tank sizes. This article describes all of the 20 gallon fish tank types and gives a review on the standard 20 gallon aquarium.
20 gallon fish tank

20 Gallon Long Fish Tank

This aquarium is typically sized at approximately 30” wide x 12” deep x 12” tall. When people or stores refer to a 20 gallon they are usually referring to this size.

20 Gallon High Fish Tank

The 20 gallon high fish tank is approximately 24” wide x 12” deep x 16” tall. It is 4” more narrow than the 20 gallon long tank and 4” taller.

20 Gallon Extra High Fish Tank

This aquarium is twice as tall as the normal 20 gallon long fish tank coming in at 20” wide x 10” deep x 24” tall. This tank is can really be described as two 10 gallon fish tanks stacked on top of each other because it comes in around the same dimensions.

20 Gallon Fish Tank Guidelines

20 gallon fish tanks are many people’s first, second, or “forever” fish tank because they are still a manageable size but they give you that much more room than a standard 10 gallon fish tank. Here are a few specs or guidelines on a standard, 20 gallon long fish tank if you are considering purchasing one:

Number of Fish

If you follow the typical rule of thumb that you can hold 1” of fish for every gallon of water than if you have fish at an average of 2” to 3” you will be able to hold about 7 fish in your new 20 gallon aquarium if it is freshwater. It will be less than that for saltwater. Please note that it is recommended you start at 25% of that capacity to begin and please test the water after 6 weeks to ensure that your aquarium is maintaining proper filtration to minimize nitrate or ammonia spikes.

Heater Size

A 20 gallon aquarium requires approximately 100 watts of heat so you would need either two 50 watt heaters or one 100 watt heater.


A 20 gallon fish tank should have a filter that pushes at least 100 GPH of water through the filter. For saltwater roof tanks the flow should be between 100-200 GPH of total water flow (between the return filter pump and powerheads or similar).


A 20 gallon fish tank should have at least 10 watts of light (for a non-planted freshwater tank) up to 100 watts of light for a full saltwater reef tank. Please note this is for standard florescent bulbs, LED lights would require much less!

As mentioned above the 20 gallon fish tank is one of the most popular in the world. If you are looking to purchase one I would recommend following the advice above if you would like to build your own 20 gallon “package” with the guidelines above or purchasing a full 20 gallon fish tank kit through the following link to Petsmart: icon