Can these two fish live together?

The question "can these two fish live together?" has no one-word answer, even though most sources give one. Two species can be perfect tankmates in a large planted tank and a disaster in a small bare one. The fish compatibility checker exists because compatibility is a set of conditions, not a verdict — and a tool that gives you a score with the reasoning behind it is far more useful than a blanket "yes" or "no." This guide explains how the checker works, how to read every part of its result, and how to turn a borderline pairing into a stable one. For the full theory, the complete fish compatibility guide goes deeper than any tool screen can.

The Four Factors the Checker Scores

Every pairing has to pass four independent tests, and the checker evaluates each as a weighted sub-score. A failure on any one can sink the pairing regardless of the others.

Water parameters — can both species actually thrive in the same temperature, pH, and hardness? The checker reads the real ranges from the species database and rewards comfortable overlap over knife-edge compromise. Temperament — will their behavior create stress, aggression, harassment, or fin-nipping? Size and predation — can one fit the other in its mouth? If so, it eventually will, no matter how peaceful it seems. Competition and habitat — do they want the same water type, the same zone, the same niche, or are they fundamentally mismatched (freshwater versus marine, coldwater versus warmwater)?

Why a single score is not enough: "compatible" means something different for two peaceful schoolers than for a fish and its live food. That is why the checker returns a relationship type alongside the number — so you know what kind of result you are looking at.

Reading the Score

The checker returns a 0–100 compatibility score with a simple interpretation. 70 and above means the pairing is generally workable — easy success with normal care. 40 to 69 means conditional: it can work, but only with the right tank size, planting, and setup, and it deserves close attention. Below 40 means the pairing is not recommended for most aquariums; find an alternative. The score is deliberately not a pass/fail line but a gradient, because that is how compatibility actually behaves.

Crucially, the score comes with the reasons behind it — the warnings and notes that explain which factor pulled it up or down. A score without reasons is just another verdict; the point of the checker is that it shows its work, so you can decide for yourself whether the conditions it flags are ones you can meet.

Relationship Types

The relationship type tells you what kind of pairing you are looking at, which changes how to read the score entirely. Tankmates is a straightforward compatible community pairing. Conditional Community works only with the right setup. Predator / Prey means one animal may hunt or suppress the other. Live Food is a special and important case: when you pair a fish with a microfauna species like daphnia or scuds, a low score is the correct answer — these are food, not tankmates, and the Live Food Encyclopedia covers that feeding relationship. Habitat Mismatch and Do Not Mix flag absolutes like coldwater-versus-warmwater or freshwater-versus-marine that no amount of tank design can fix.

The Sub-Scores and Risk Flags

Below the headline number, the checker breaks the result into sub-scores — water, temperature, behavior, predation safety, habitat, and setup ease — so you can see exactly where a pairing is strong or weak. A pairing might score well on water and temperature but poorly on predation safety, which tells you the species are chemically compatible but one is a size risk to the other. The risk flags summarize the specific hazards: temperature mismatch, fin-nipping, predation risk, listed incompatibility, and so on. Reading the sub-scores turns a single number into an actionable diagnosis.

The Recommended Setup

The most practical part of the result is the recommended setup, because tank design is the lever you control. The checker returns a minimum tank size, flow level, planting density, and hiding requirement tuned to the pairing — because many borderline combinations succeed in a well-designed tank and fail in a poorly designed one. Dense planting breaks sightlines and reduces aggression; hiding spots give subordinate fish refuge; adequate size dilutes territorial conflict. Confirm you actually have the volume and filtration for the combined stock with the stocking density calculator, and check that both species suit your water with the water parameter reference.

How to Use It Well

Run a pairing before you buy, not after. Test every new fish against every existing one, since compatibility is about the whole community, not isolated pairs. Read the warnings, not just the score — a 65 with a fin-nipping flag means something specific you can design around. Treat "Conditional" results as a checklist: meet the setup conditions and the pairing moves toward success; ignore them and it moves toward failure. And when a result surprises you, read the full compatibility guide to understand the principle behind it. This explain-the-why approach is the foundation of every tool in SpawnOS, by Blackwater Aquatics Canada.

Worked Examples From the Checker

A few real pairings show how the score and relationship type work together. Run two male bettas and the checker returns a very low score with a "Species-Only Conflict" flag — territorial aggression caps the result no matter how perfectly their water parameters match, because behavior overrides chemistry here. Run a betta and cherry shrimp and you get a mid-range "Conditional" score with a predation-risk note: the pairing can work in a densely planted tank with an individual betta of calm temperament, but the colony's babies are always at risk, so the recommended setup leans on heavy cover. Run a neon tetra and cardinal tetra and the score is high with a clean "Tankmates" verdict — same soft warm water, same peaceful schooling behavior, no size gap. And run a betta and daphnia and the checker deliberately scores it low as "Live Food," because that is a feeding relationship, not a community — exactly the answer a breeder wants.

The pattern across all four: the number tells you how easy success will be, the relationship type tells you what kind of result you are looking at, and the warnings tell you the specific condition to manage. Reading all three together is how you turn the checker from a verdict machine into a planning tool.

From Checker to Stocked Tank

The checker evaluates pairs, but a community is more than a set of pairs — it is the whole group sharing one set of conditions. Use the checker as one step in a sequence: pick a target water chemistry from the water parameter reference, test each candidate species against every existing one (not just the new pair), confirm the combined load fits with the stocking density calculator, and meet the setup conditions the checker recommends before you buy. A community planned this way — every pairing checked, the conditions met in advance — behaves far more predictably than one assembled by impulse at the store. That shift, from buying fish that "look good together" to building a community with reasons, is the entire point of treating compatibility as conditions.

Frequently Asked Questions

How does the fish compatibility checker work?

It scores two species across four factors — water parameters, temperament, size and predation, and habitat and competition — using real parameter and care data, then returns a 0–100 score, a relationship type, sub-scores, risk flags, and a recommended setup, all with the reasoning behind them.

What does the compatibility score mean?

70 and above is generally workable; 40 to 69 is conditional and needs the right setup; below 40 is not recommended for most tanks. The score is a gradient, not a pass/fail line, and it comes with warnings that explain it.

Why does my fish-and-daphnia pairing score so low?

Because that is a feeding relationship, not a tankmate pairing. The checker labels it "Live Food" — the fish will eat the daphnia, which is the intended outcome. A low score there is correct, not a warning.

Is the compatibility checker free?

Yes. The fish compatibility checker and every SpawnOS calculator are free and require no account.