What Size Fish Tank Is Best for a Beginner?

Why bigger is often easier

Small tanks look like the gentle option. Sources say the opposite, for three reasons:

• Stability. A welfare charity and a trade association both say larger volumes hold steadier temperatures and water conditions.
• Room for error. One retailer says more water means more room for error, and that small tanks are less forgiving of overstocking.
• Slower swings. The same retailer says smaller tanks are more open to temperature swings, and they have a greater need for regular water changes.

A veterinary manual says very small tanks may need as much as 50% of their water changed each week.

5 gallons or a 20-gallon long?

Here is what our sources support, and where they stop.

Our advice: if you have the space and budget, choose a 20-gallon long over a 5-gallon for a first tank. A 10-gallon is a reasonable step down. A 5-gallon can work if you pick from its short list of species and you are happy to keep up with water changes.

On one maker’s chart, the 20-gallon long has nearly three times the surface of the 5.5-gallon, and about a quarter more than a 20-gallon high of the same volume.

Long or tall?

Two tanks can hold the same water and suit very different fish.

• Long tanks give more surface. A welfare charity says the surface is where oxygen enters, and suggests adding an air stone to help. A veterinary manual also measures freshwater stocking by surface area, not by gallons.
• Tall tanks suit tall fish. One retailer recommends more height for vertically shaped fish such as angelfish.
• Long tanks give more swimming length. The same retailer points to a 20-gallon long for fish that need room to swim back and forth.

Our advice: unless your species needs height, choose the longer tank. It also gives bottom dwellers more floor.

Size, shape and weight

These figures come from one manufacturer’s chart, so other brands will differ. Length, width and height are in inches, rounded.

The chart notes that filled weights vary with gravel and decorations.

How size limits your fish

Tank size is the first thing that rules species in or out. Here is how many of our species fit at each size:

Counted from our species list when this page was built, using each species' minimum tank size. A tank within 10% of a listed minimum counts as a fit, the same allowance our lists use. Fitting the minimum is not the whole answer: group size and tank mates still count.

Use a species’ adult size. One retailer points out that most fish are sold as juveniles. A trade association advises buying a tank that can hold them fully grown, or being ready to upgrade. A welfare charity warns that some fish, such as giant gourami and pacu, grow too large to be suitable pets.

Why the old stocking rules are only a start

Three rules of thumb give three different answers:

• A veterinary hospital group gives 1 gallon of water per inch of fish. One retailer says that mainly applies to small community fish.
• A veterinary manual gives 1 inch of fish per 12 square inches of surface for freshwater.
• A welfare charity gives 1.5 to 2 liters of water per centimeter of tropical fish, and says to treat that only as a guideline.

The veterinary manual adds that the true limit depends on filtration and territory, and testing the water shows how the filter is coping. So use each fish page for its minimum tank and group size, stock lightly, and add fish slowly. Our stocking checker tests a plan against those figures.

The cost

• Buying it. One retailer says cost is the main reason people start small, since the parts for a small tank cost less.
• Running it. One keeper site treats heating as the main running cost. It estimates very roughly $1.50 a month per 10 gallons, to heat a tank 10°F above the room. That was priced in 2018, so your figure will differ. It also says larger tanks cost less per gallon.

We have no independent figures for the total cost of a setup, so we give none.

Choose in five steps

1. Measure the spot

   Keep it out of direct sunlight and away from radiators, and within reach of grounded outlets. Sun can overheat the water.

2. Check the floor and stand

   Use the weights above as a guide, and allow extra for gravel and decorations.

3. Test your tap water

   Your water narrows your species as much as your tank does.

4. Pick the species, then the tank

   Start with our 20-gallon list, or 10 gallons and 5 gallons if space is tight.

5. Budget for the gear and the wait

   You need a filter and a heater, and time to cycle the tank before any fish arrive.