20-Gallon Aquariums: What You Need to Know

Twenty-gallon fish tanks are many people’s first, second, or “forever” fish tank because they are a manageable size but offer more room than a standard 10-gallon aquarium.

Here's what you need to know about 20-gallon fish tanks.

Types of 20-Gallon Aquariums

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 tank, 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" narrower 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 can be thought of as two, 10-gallon fish tanks stacked on top of each other.

How Many Fish can Live in a 20-Gallon Aquarium?

If you follow the typical rule of thumb (you can hold 1 inch of fish for every gallon of water), then you will be able to hold about 7 fish that average 2-3 inches in length inside your 20-gallon aquarium, provided it's a freshwater tank. It will be less than that for saltwater. It is recommended you start at 25% fish capacity to begin with. Test the water after 6 weeks to ensure proper nitrate or ammonia levels. Only then add the rest of the fish. 

What Heater Size is Right for a 20-Gallon Aquarium?

A 20-gallon aquarium requires approximately 100 watts of heat, so you would need either two, 50-watt heaters; or one, 100-watt heater. We recommend purchasing a titanium aquarium heater to avoid some common issues with glass heaters, such broken glass or errant voltage.

GPH for 20-Gallon Fish Tank Filters

A freshwater 20 gallon fish tank should have a filter that runs at least 100 GPH. For saltwater roof tanks, the flow should be between 100-200 GPH (beween the return filter pump and powerheads or similar).

Lighting for 20-Gallon Aquariums

A 20-gallon fish tank should have at least 10 watts of light (for a non-planted, freshwater tank) and up to 100 watts of light for a full saltwater reef tank. This is for standard florescent bulbs; LED lights would require much less! Exercise caution if you plan to use a metal halide fixture, as those have a tendency to get pretty hot. A 20-gallon fish tank doesn't take much to heat up.