Using the Tank Builder

The Tank Builder is a free planner that checks whether the fish you want can live together in your tank. You set a size, add fish, and it instantly shows a compatibility score, every problem it finds in plain English, how full the tank is, the heater and filter you need, and tankmates that would fit. It works without an account. Signing in lets you save tanks, add photos and log water tests.

What's on the Tank Builder page?

The page at /tank-builder is laid out top to bottom like this:

• Header: the title Tank Builder, plus two buttons, Share build and New.
• Your tanks: a row of cards for your saved tanks (only when you're signed in and have saved at least one).
• The tank picture: an animated drawing of your tank with your fish in it, with small labels on top showing the size, how many animals and species you have, how full the tank is, and your score.
• Build column (left on wide screens, first on phones): 1. Tank size, 2. Add fish (with Tankmates that fit underneath), and 3. Save your build (or a Sign in to save box if you're signed out).
• Results panel (right on wide screens, below on phones): two tabs, Compatibility and Water test.

Below the builder there's a short How the Tank Builder works section, a Heater and filter size by tank size table linking to each tank size guide, an Aquarium FAQ, and Keep going links to Browse fish species, Check your water, Aquarium glossary and Buy fish locally.

On a phone, a See results link appears next to 2. Add fish once you've added a fish. Tap it to jump down to the results.

Do I need an account to use the Tank Builder?

No. Everyone can set a size, add fish, read every result, share a build and use the Water test tab without signing in. Your unsaved build is kept on the device you're using, so it comes back if you refresh or leave and return.

You need to be signed in to:

• Save tanks (up to 4)
• Add photos to a tank
• Show a tank on your profile and in the feed
• Log water test readings to a tank

When you're signed out, the save area says "Your build is kept on this device. Sign in to save it, add photos, share it on your profile and track water tests." Tap Sign in to save to sign in; you come back to the Tank Builder afterward. See Saving and sharing tanks.

How do I set my tank size?

Use the 1. Tank size box. You have three ways to enter it:

1. Type it. Enter the number in the size box. The placeholder shows "e.g. 29" in gallons or "e.g. 110" in litres. Decimals are fine.
2. Tap a common size. The quick buttons are 5, 10, 20, 29, 40, 55, 75 and 125 gal. Tapping one switches the builder to gallons and highlights that button.
3. Measure it. Tap Measure it (see the next section).

To switch units, use the Gallons / Litres toggle at the top right of the size box. The number you typed converts automatically (rounded to one decimal place). Heater and filter results are always shown in watts and gallons per hour, and the builder always works internally in US gallons.

A blank box or 0 means "no size yet." You can still add fish and see compatibility problems, but stocking level, tank-size checks, and heater and filter sizes only appear once there's a size. The picture label shows No size yet until you enter one.

If the size you type is within about 15% of one of our tank size guides, a link appears under the box, for example "29 gallon tank guide: stocking ideas, heater and filter sizes." See Tank size guides.

How do I work out my tank's volume by measuring it?

1. In 1. Tank size, tap Measure it. A small panel opens.
2. Enter the inside length, width and water height (not the glass height). In gallons mode these are in inches; in litres mode they're in centimetres. The panel reminds you: "Inside length × width × water height, in inches" (or centimetres).
3. As soon as all three boxes have a number, a button appears, for example Use 28.1 gal or Use 106 L. Tap it to fill in the size. The panel closes.

The math: inches multiplied together and divided by 231 gives US gallons. Centimetres multiplied together and divided by 1,000 gives litres. Tap Measure it again to close the panel without using the result.

Measuring the inside and the actual water line gives a smaller, truer number than the size printed on the box. For example, a tank sold as a 29 gallon holds about 28 gallons of water.

How do I add fish?

1. Tap the search box under 2. Add fish (placeholder "Search fish, shrimp, snails…").
2. With nothing typed you'll see Popular picks, up to 8 well-known fish you haven't added yet.
3. Type any part of a common name, another name the fish goes by, its scientific name, or a group (like "shrimp" or "cory"). Up to 10 results show, best matches first.
4. Tap a result to add it. On a keyboard you can use the arrow keys to move and Enter to add; Escape closes the list.

Each result shows the fish's group, adult size in inches and recommended minimum tank (in the style "[group] · 1.5" · 10+ gal"). If you've entered a size and the fish needs a bigger tank, the result shows an amber Needs 40+ gal style tag instead of a plus sign. You can still add it to see what the builder says.

Fish you've already added don't appear in search results, so you can't add the same species twice. To change how many, use the quantity controls in your list instead. The X at the end of the search box clears what you typed.

The builder searches fish, shrimp and snails from our species library; the "How the Tank Builder works" section below the builder shows how many species it currently knows.

How many of each fish get added?

Schooling and shoaling fish come in at their minimum group size (for example 6). Everything else comes in as 1. Tankmate suggestions come in at the quantity shown on the suggestion.

How do I change quantities or remove a fish?

Each fish in your list has controls on the right:

• - and + lower or raise the count by one.
• The number box lets you type a count directly.
• X removes that species from the build.

Quantities go from 1 to 999. The minus button stops at 1; to get to zero, use X.

Under each name you'll see its adult size (for example "2" adult"), its recommended minimum tank ("20+ gal"), and, if you have fewer than its minimum group, "keep 6+" with an amber border around the number box.

A fish row turns amber when that species is involved in a Conflict or Check. Tap a fish's name to open its care guide in a new tab (see Reading a species page).

What does the tank picture show?

The picture at the top draws your fish in the part of the tank where they actually swim (top, middle or bottom, with shrimp and snails on the sand), sized by how big they grow and colored roughly like the real fish. It's the quickest way to spot an empty bottom, a crowded middle, or one huge fish among tiny ones.

• It draws up to 7 of each species (9 if you have more than 20 of one), so big schools don't fill the screen.
• Fish involved in a Conflict or Check get a soft glow. Tips don't cause a glow.
• As the tank passes 90% stocked the water takes on a faint green-brown tint, and it gets stronger past 130%.
• With no fish it says "Your fish will swim here."

The labels on the picture show your size (for example "29 gal · 110 L"; in litres mode it shows the gallon figure only), "12 animals · 3 species", "42% stocked" (only when a size is set), and your score, for example "88/100 · Good match." The score label is green, amber or red to match the results.

How do I read the Compatibility tab?

The Compatibility tab is the default view in the results panel. Before you add fish it says "Add fish to get your score." Once you add one:

1. Score dial: a ring that fills with your score out of 100, with a label such as Great match or Needs changes. Next to it are two counters: how many conflicts and how many items to check.
2. The problem list: every issue the builder found, worst first. Each one is tagged Conflict (red), Check (amber) or Tip (blue info icon), with a title that names the fish and a plain-English explanation. If there are none you'll see "No problems found with this combination. Every fish has its own personality, so watch the first few weeks."
3. How full is it?: your stocking percentage, a label (Lightly stocked, Comfortably stocked, Near capacity or Overstocked) and a bar with a marker at 90%.
4. Gear for [size] gallons: Heater watts, Filter flow in GPH, and Set to (the heater temperature).
5. Water each fish likes: a chart of each fish's temperature and pH range with the shared range highlighted. It opens automatically when you have two or more species.

If you haven't entered a size, items 3 and 4 are replaced by "Add your tank size to see how full it is and what heater and filter you need."

Every rule, severity and score cutoff is explained in Tank Builder compatibility warnings explained. The stocking math and gear formulas are in Stocking level, heater and filter sizing.

What does the "Water each fish likes" chart show?

Open Water each fish likes at the bottom of the Compatibility tab. There are two charts:

• Temperature, on a scale from 60 to 90°F
• pH, on a scale from 5 to 9

Each fish gets a colored bar showing its comfortable range (the colors match the dots in your fish list). When you have two or more fish, the range they all share is highlighted in green and labeled, for example "Shared: 74-78°F." If there's no range they all share, the label reads No overlap in red, which is the same problem the builder flags as a temperature mismatch or pH difference.

Fish with no temperature or pH data in our library are left off the chart. Hardness (GH) isn't charted.

How do tankmate suggestions work?

Once you've set a size and added at least one fish, Tankmates that fit appears under your fish list with up to 6 suggestions. Each shows the quantity (for example "6 × [fish name]") and a reason:

• "Fills the top of the tank" / "Fills the middle of the tank" / "Fills the bottom of the tank"
• "Easy keeper, no conflicts"
• "No conflicts with your fish"

Tap a suggestion to add it at that quantity. The list updates every time your build changes. How suggestions are picked is covered in Stocking level, heater and filter sizing.

What if the fish I want isn't in the Tank Builder?

If your search finds nothing, the list says "“[what you typed]” isn't in our library yet." with a Request it and earn bubbles button. If you get results but not the fish you meant (and you typed at least 3 characters), tap Not the one? Request “[what you typed]” at the bottom of the list.

Either button opens Request a fish: "We'll add it to the library and the Tank Builder. You get bubbles and a trophy when it's in." The name you typed is filled in. Fill in:

• Name you know it by (required, up to 80 characters). Leaving it empty shows "Enter the fish's name."
• Scientific name, if you know it (optional, up to 100 characters)
• Anything that helps us find it (optional, up to 500 characters)

Tap Send request. You must be signed in to request; signed out, the box shows a Sign in link instead. More in Requesting a species.

What do the Share build and New buttons do?

• Share build creates a link to your current size and fish that anyone can open, no account needed. On a phone it opens your share menu; on a computer it copies the link and shows "Link copied. Anyone with it can open this build." The button is grayed out until you've entered a size or added a fish. Full details in Saving and sharing tanks.
• New clears the builder for a fresh start: size, fish, tank name, photos, public setting and any water test values. If you're editing a saved tank with changes you haven't saved, it asks "You have unsaved changes. Start a new build anyway?" first. New does not delete any saved tank.

Can I check my water in the Tank Builder?

Yes. Switch the results panel to the Water test tab and enter your test kit numbers (Temperature, pH, Ammonia, Nitrite, Nitrate, GH, KH). You get the same plain-English read as Water Check, plus two extra checks because the builder knows your fish: whether your pH and your temperature fall inside the range your fish share.

Signed in, you can log the reading to a saved tank to build a history. See Water Check, Water test results explained and Logging water tests.

Is the Tank Builder on the size guide pages the same tool?

Yes. Each tank size guide (like the 29 gallon guide) has the full builder under the heading "Build your own 29 gallon tank," already set to that size. Everything works the same, including saving and logging, with two differences:

• New resets the size back to that guide's size, not to blank.
• An unsaved build on a guide page is not kept on your device. If you refresh or leave, it's gone. Save it, share it, or tap Open in builder on a stocking idea to continue on the main Tank Builder page, where drafts are kept.

How accurate is the Tank Builder?

The builder uses the care data in our species library: adult size, recommended minimum tank, temperature and pH range, water type, temperament, schooling minimum, where it swims, diet and whether it nips fins. Its limits are deliberately cautious, and its advice is a guide, not a guarantee. Individual fish vary, and filtration, plants, aquascaping and maintenance all change how a real tank behaves. Watch new additions closely for the first few weeks.

If a species' data is missing, the builder simply can't check that part (for example, no temperature range means no temperature check for that fish). If you think a species' data is wrong, email support@undergroundaquarium.com with the species name and what looks off.

Common problems

My fish count, score or gear results aren't showing. Add at least one fish. The score, problem list and gear only appear once there's a fish in the build. Stocking and gear also need a tank size.

A fish I added isn't showing in search anymore. Fish already in your build are hidden from search. Find it in your list and change the number there.

I can't set a quantity to 0. The lowest count is 1. Tap the X on that row to remove the fish.

My build disappeared. Unsaved builds are kept only in the browser on that device, and not at all on tank size guide pages. Private or incognito windows, clearing site data, or switching devices lose them. Opening someone's shared build link also replaces your unsaved build on that device. Sign in and save tanks you want to keep.

I switched to litres and the numbers changed slightly. Converting rounds to one decimal place, so switching back and forth can shift the last digit. Saved tanks are always stored and reopened in gallons.

The builder says my fish can't live together, but they're fine in my tank. The builder is cautious on purpose, and individual fish vary. Read the explanation on each warning; many are Checks or Tips meaning "workable, but watch this." See Tank Builder compatibility warnings explained.

Tapping Share build does nothing. The button is disabled until you enter a size or add a fish. If you see "Couldn't copy the link," your browser blocked clipboard access; try again from a different browser or copy the link from your phone's share menu.