Every beginner has stood at a fish store, looked at two species in different tanks, and wondered if they can live together. The answer almost never comes down to one factor. Compatibility in an aquarium is six separate questions stacked on top of each other, and any one of them can sink an otherwise reasonable stocking.

This guide walks through each question, the most common mistakes, and how to think about compatibility before you bring fish home.

The six questions to ask about every pairing

Before you put two species in the same tank, run through this checklist. If any answer is "no," it's a hard pass.

1. Same water type?

Freshwater fish live in fresh water. Saltwater fish live in salt water. Brackish species sit in between. You cannot mix them. This sounds obvious, but stores sometimes display tanks of mixed origin, and beginners assume the mix will translate at home. It does not.

The builder enforces this automatically. Pick freshwater or saltwater up front, and the species library filters to match.

2. Do their temperature ranges overlap?

Tropical freshwater is roughly 75 to 82°F. Coldwater (goldfish, white cloud minnows, some loaches) sits at 60 to 72°F. A tropical fish in a goldfish tank slowly stresses out and dies. The other direction is just as bad.

Even within tropical, ranges are tighter than people think. Discus want 84 to 86°F. Most tetras top out around 80°F. Putting them together means one species lives at the edge of its comfort zone every day.

Look up the actual minimum and maximum for both species and confirm the overlap is at least 4 degrees wide. If it isn't, pick a different pairing.

3. Are their adult sizes compatible?

The rule of thumb: if a fish can fit another fish in its mouth, it eventually will. "Eventually" might be six months after both go in the tank as juveniles.

Mature angelfish can swallow a neon tetra. Adult Oscars eat anything under 4 inches. Bichirs, larger cichlids, and most predatory catfish are the same story. Always look at the species' adult size, not the size at the store.

4. Do their temperaments match?

Aquarium species fall into three rough buckets: peaceful, semi-aggressive, and aggressive. Mixing across two buckets is risky. Mixing across all three almost never works.

A few specific patterns to watch:

• Fin-nippers (tiger barbs, serpae tetras, some danios) destroy long-finned tankmates. Bettas, angelfish, and gouramis come home from the store with shredded fins.
• Territorial cichlids (most New World, all African) need either species-only setups or carefully matched tankmates with similar temperament and size.
• Solitary species like bettas don't enjoy company even from peaceful tankmates. Plan around them, not with them.

5. Are the schooling needs respected?

Many "peaceful community fish" only stay peaceful in groups. Tetras, rasboras, danios, corydoras, and many barbs need at least 6 of their own species in the tank to feel safe. Single them up and they get stressed, hide, lose color, and pick on each other.

Don't add three of one tetra and three of another and call it a school. They need 6 or more of the same species.

6. Do their tank zones overlap problematically?

Fish use different vertical levels of the tank: top, middle, and bottom. A community works best when species spread across these zones rather than competing for the same one.

Three middle-zone schoolers crowded together will compete and stress out. A tetra school plus a bottom-feeder species like corydoras plus a single mid-water centerpiece (gourami, ram cichlid) uses the whole tank well.

Common compatibility mistakes

These come up constantly in beginner builds:

Putting one of a schooling species in a community. Solo tetras, danios, and corys are stressed. Always six or more.

Mixing male and female livebearers without a plan. Guppies, mollies, and platies breed every month. The fry overwhelm small tanks within a year.

Adding small shrimp to a tank with adult community fish. Most community fish eat baby shrimp. Adult cherry shrimp survive in tanks with small peaceful species, but the colony won't grow. Amano shrimp are big enough to coexist. Cherries are not.

Plant-eating species in a planted tank. Goldfish, silver dollars, and many cichlids destroy live plants. Stock around the plants you want, not against them.

Reef-safe assumptions. "Reef-safe" is species-specific, not category-wide. Many wrasses, angels, and butterflyfish nip coral or eat invertebrates. Always check each species individually before adding to a reef tank.

Wrong sex ratios in cichlids. Most cichlids need specific male-to-female ratios, often 1:3 or higher. Adding "a pair" of mbuna usually ends with one dead female within a week.

When the rules bend

Experienced keepers sometimes break compatibility guidelines and get away with it. A betta in a 20-gallon community with peaceful schoolers occasionally works. A tiger barb school in a tank without long-finned fish stays peaceful enough. A semi-aggressive cichlid can sometimes coexist with similarly-sized tankmates if it's introduced last.

Two things to know about these exceptions:

1. They're case-by-case judgments, not species-level rules. The same betta that ignored its tankmates for a year may suddenly start fighting.
2. Have a backup plan. Either a second tank you can move fish to, or a return arrangement with the store.

Use the builder to pre-check stocking

The tank builder runs all six checks above on every species you add and surfaces conflicts as caution, warning, or critical alerts. Drag a species in and the system tells you whether it fits the water type, temperature, pH, size, and temperament profile of what's already there.

It's not a substitute for research, but it catches the obvious mistakes before you've spent money on fish that won't survive together.

Before you stock

Compatibility is one of three things to verify before you add a fish to your tank. The other two:

• The tank is fully cycled. See How to cycle your aquarium.
• The species fits the tank size. For 10-gallon stocking ideas, see How to stock a 10-gallon planted tank.

Get all three right and most beginner mistakes never happen.