Tank size guides

Every common aquarium size has its own guide page at /tank-builder/[size]-gallon, for example the 29 gallon guide. Each guide gives the tank's dimensions, gear sizes and weight, three ready-made stocking ideas that pass the Tank Builder's checks, a built-in Tank Builder set to that size, and lists of popular fish that fit and fish that need more room. They're free and need no account.

Which tank sizes have a guide?

There are 12 guides:

• 5 gallon
• 10 gallon
• 15 gallon
• 20 gallon
• 29 gallon
• 40 gallon
• 55 gallon
• 75 gallon
• 90 gallon
• 125 gallon
• 150 gallon
• 180 gallon

Other sizes (like 30, 36 or 65 gallons) don't have a guide page. For those, type your size into the Tank Builder, which gives the same heater, filter and stocking numbers for any size.

How do I find the size guides?

• On the Tank Builder page, scroll to Heater and filter size by tank size and tap a size in the Tank column.
• In the Tank Builder, when the size you type is within about 15% of a guide size, a link appears under the size box, for example "29 gallon tank guide: stocking ideas, heater and filter sizes."
• At the bottom of every guide, Other tank sizes lists all 12, with previous and next links (for example "← 20 gallon" and "40 gallon →").

What's on a tank size guide page?

From top to bottom:

1. Breadcrumb: "Tank Builder › 29 gallon tank." Tap Tank Builder to go back.
2. Title: "Tank size guide" above "29 Gallon Tank: Stocking Ideas, Heater & Filter Size," with a one-line description of the size.
3. Fact tiles: Dimensions, Volume (gallons and litres), Heater, Filter flow, Filled weight, Gravel (2" bed), Weekly water change, and Water weight.
4. [Size] gallon stocking ideas: up to three stocking plans, each with an Open in builder link.
5. Try it yourself: Build your own [size] gallon tank: the full Tank Builder, already set to that size.
6. Fish that fit a [size] gallon tank: up to 24 popular freshwater species.
7. Popular fish that need more than [size] gallons: up to 8 species.
8. [Size] gallon tank FAQ: five questions and answers.
9. Other tank sizes: links to every guide.

Each part is explained below.

What sizes, dimensions and descriptions do the guides use?

Dimensions are the usual US glass tank footprint, length × width × height in inches. Brands vary a little, so measure before buying a stand or lid.

• 5 gallon, 16" × 8" × 10": a nano tank, for a single betta, a shrimp colony or a small group of the tiniest fish.
• 10 gallon, 20" × 10" × 12": the classic starter tank; small enough for a desk, but too small for most schooling fish to really swim.
• 15 gallon, 24" × 12" × 12": a 2-foot tank with room for a nano community.
• 20 gallon, 24" × 12" × 16". The guide also mentions the 20 long (30" × 12" × 12"), which gives fish more swimming length.
• 29 gallon, 30" × 12" × 18": a very popular community tank, tall enough for angelfish-shaped fish.
• 40 gallon, 36" × 18" × 16" (a 40 breeder). The guide notes a 40 long is 48" × 12" × 16".
• 55 gallon, 48" × 13" × 21": the 4-foot classic.
• 75 gallon, 48" × 18" × 21": same length as a 55 but much deeper front to back.
• 90 gallon, 48" × 18" × 24": a tall 4-foot tank.
• 125 gallon, 72" × 18" × 21": a 6-foot tank.
• 150 gallon, 72" × 18" × 28": a tall 6-footer.
• 180 gallon, 72" × 24" × 25": a 6-foot tank that's 2 feet deep.

These are outside glass measurements. The water inside is a bit less than the size on the label: if you measure your own tank with the Tank Builder's Measure it, use the inside length, width and water height.

What are the heater, filter and weight numbers for each size?

These are the figures shown on each guide's fact tiles:

How they're worked out:

• Heater: 3 to 5 watts per gallon, rounded to the nearest 5 W. From 75 gallons up, the FAQ recommends two smaller heaters at opposite ends.
• Filter flow: 4 to 6 times the tank volume per hour. For messy fish or a busy tank, the FAQ says to go up to about 8 times.
• Water weight: 8.34 lb per gallon.
• Filled weight: roughly 10 lb per gallon, allowing for glass, gravel and decor. Use a stand made for aquariums.
• Gravel: a 2 inch bed across the tank's footprint.
• Weekly water change: 25% of the tank's volume.

The Tank Builder calculates gear for your exact fish too, which can raise the filter range. See Stocking level, heater and filter sizing.

How are the stocking ideas chosen?

Each guide shows up to three ready-made plans, built automatically from our species library and checked by the same engine the Tank Builder uses. Every plan:

• Has no Conflicts and no Checks (a Tip or two is allowed).
• Stays at or under 80% stocked.
• Uses only freshwater species that have temperature, adult size and minimum tank data, whose minimum tank fits the size, and that aren't marked "not recommended," "expert" or "advanced."
• Leaves out goldfish, coldwater fish and African Rift Lake cichlids, which need tanks built around them rather than a community mix.

Plans are built around a lead fish, with supporting fish added by tank size: small tanks under 10 gallons lead with a single centerpiece fish; 10 gallons and up lead with a school and add a bottom dweller; 20 gallons and up can add a centerpiece; 40 gallons and up add a second school; 75 gallons and up add another centerpiece. Shrimp or a snail may be added last. Schools are then grown (up to twice their minimum, or three times in tanks over 60 gallons) while the tank stays at 70% stocked or less.

Each plan gets a new lead fish, so the three plans differ. On tanks of 55 gallons and up, fish that actually need the space are tried first; smaller tanks lead with popular classics.

How do I read a stocking idea card?

Each card has:

• A title named after its lead fish: "[Fish] community" (a school leads), "[Fish] centerpiece" (a single fish leads) or "[Fish] colony" (shrimp lead).
• A checklist of each species with its quantity, for example "8 × [fish name]." Tap a name to open its species page.
• A green badge with its stocking level, for example "46% stocked."
• Open in builder, which opens the main Tank Builder with that size and those fish loaded, so you can tweak it, save it or share it.

The ideas are generated from our current species data and refresh periodically, so they can change over time as the library grows.

Why does a guide show fewer than three stocking ideas, or none?

Plans are only shown when the engine can build a clean one. Small tanks have fewer fish that fit without warnings, so a guide may show one or two ideas. If none can be built, the section is hidden and the FAQ says the tank "is best kept very lightly stocked: a single small fish, or a shrimp and snail colony." Use the built-in Tank Builder on the page to check your own plan.

What are "Fish that fit" and "Popular fish that need more"?

• Fish that fit a [size] gallon tank: up to 24 popular freshwater species whose recommended minimum is that size or less, most popular first. Each shows adult size, minimum tank and group size, in the style "1.5" · 10+ gal · groups of 6+." These fit by size only; they aren't checked against each other. The page reminds you: "Check they get along in the builder before you buy."
• Popular fish that need more than [size] gallons: up to 8 popular freshwater species whose minimum is bigger, shown as amber chips with their minimum, for example "[fish name] (30+ gal)."

Tap any fish to open its species page.

What questions does the FAQ on each guide answer?

Every guide has the same five questions, answered for that size:

1. How many fish can go in a [size] gallon tank? It uses the first stocking idea as an example with its stocking percentage, and explains that small, tidy fish let you keep more while messy fish fill a tank faster.
2. What size heater do I need? The wattage range, going toward the top if the room runs cool, and the two-heater advice from 75 gallons up.
3. What size filter does it need? The GPH range and the higher figure for messy fish.
4. What are the dimensions? The standard size, plus the alternate shape for the 20 and 40 gallon.
5. How much does it weigh? Water weight and the rough filled weight.

How is the builder on a guide page different from the main Tank Builder?

It's the same tool, already set to that guide's size, under the heading "Try it yourself: Build your own [size] gallon tank." You can add fish, see every result, save, share and log water tests there. Differences:

• New resets back to the guide's size rather than a blank box.
• An unsaved build on a guide page isn't kept on your device; refreshing or leaving loses it. Save it, share it, or tap Open in builder on a stocking idea to work on the main Tank Builder page, where your draft is kept.
• The "tank guide" link under the size box doesn't appear when you're already on that size's guide.

See Using the Tank Builder.

Common problems

My tank is 30 or 36 gallons and there's no guide. Only the 12 sizes listed have guides. Type your exact size into the Tank Builder for the same calculations.

My tank's measurements don't match the guide. Brands vary slightly, and the guide shows outside glass dimensions. Measure your own tank.

The stocking ideas changed since I last looked. They're generated from our species data and refresh periodically. To keep one, tap Open in builder and save it (signed in) or share the link.

A fish in "Fish that fit" gets a warning in the builder. That list is by tank size only. Compatibility with your other fish is checked only in the builder.

I lost my build on the guide page. Builds on guide pages aren't kept as drafts. Use Open in builder, or build on the main Tank Builder page, and save or share what you want to keep.