How to Use Fish That Fit (2-Minute Guide)

Fish That Fit is built to give you a useful answer in well under a minute, but if you want to get the most out of it, and understand why it's telling you something, here's the full walkthrough.

Step 1: Pick your water type

The first toggle is Freshwater or Saltwater. This changes everything downstream: the species list, the compatibility rules, and the bioload target (saltwater is held to a more conservative standard because reef tanks are less forgiving).

Switching water type clears your current fish list, since you can't mix the two in one tank. So choose this first.

Step 2: Pick your units and tank size

Not in the US? Flip the Units toggle to Metric and the whole tool switches to litres, centimetres, and degrees Celsius. Everything below works the same in either system, so use whatever you think in.

Then type your tank volume. One thing people get wrong here: use the net water volume, not the number on the box. Substrate, rock, and decor displace water, and the tank is never filled to the absolute brim. A "20-gallon" (about 75-litre) tank in practice holds more like 17 to 18 gallons of water. If you want to be precise, subtract roughly 10% from the rated size.

Step 3: Set your filtration

The filtration dropdown has three settings:

• Light: a single sponge filter or a small hang-on-back unit.
• Standard: filtration rated for your tank size. Most setups.
• Heavy: an oversized canister filter or a sump.

Stronger filtration means more surface area for beneficial bacteria, which lets the tank safely carry a bit more bioload. Fish That Fit adjusts your capacity accordingly. When in doubt, leave it on Standard.

Step 4: Add your fish

Use the search box to find species by common or scientific name, then hit Add. A couple of helpful touches:

• When you add a schooling fish, it's added as a proper group automatically (for example, neon tetras come in as a school of six), because keeping them in smaller numbers is a mistake the tool is trying to steer you away from.
• You can fine-tune any species' quantity with the + / − steppers, or remove it entirely.

Add the fish you already own and the ones you're considering. That is the whole point. The tool is most useful as a "what if" before you go shopping.

Step 5: Read your results

The right-hand panel updates live. Here's how to interpret it.

Bioload percentage

This is your headline number: how full your tank is in terms of waste load, not just water volume.

• Under 70%, Lightly stocked. Room to add more.
• 70 to 85%, Well stocked. A healthy, balanced load with a safety margin. This is the sweet spot.
• 85 to 100%, Fully stocked. Near capacity. Keep up with water changes and don't add much more.
• Over 100%, Overstocked. Expect water-quality swings. Rehome, upgrade the tank, or boost filtration.

Compatibility report

Below the gauge you'll get plain-English warnings, colour-coded by severity:

• Red (⛔) is a hard problem, most often mixing freshwater and saltwater species.
• Amber (⚠️) is a real conflict to address: aggression, fin-nipping, predation, a tank that's too small, a school that's too small, or a temperature clash.
• Blue (ℹ️) is a heads-up worth knowing, like a minor pH-preference mismatch.

If everything checks out, you'll see a green all-clear instead.

Fish that would still fit

Finally, the tool suggests compatible species that fit in your remaining capacity, already filtered for temperament, water parameters, and group size. Click any suggestion to drop it straight into your plan.

A couple of tips

• Plan before you buy. The best time to use Fish That Fit is at the kitchen table, not standing in the fish-store aisle making a snap decision.
• Treat it as guidance, not gospel. Individual fish have personalities, and your aquascape matters. The tool gets you a well-informed starting point; your observation does the rest.
• Stock slowly regardless of the number. Even a green 75% result should be built up over several weeks so your filter can keep pace.

That's the whole tool. New to the hobby? Pair this with how to cycle a new aquarium and you'll avoid the two biggest beginner pitfalls in one sitting.