Can freshwater and saltwater fish live in the same tank?

No. A freshwater fish has body fluids saltier than its water, so water keeps entering through the gills and the kidneys work to expel it; in seawater that reverses and the fish loses water and dies within hours. A marine fish does the opposite and swells in fresh water. The two salinity requirements point in opposite directions, and no middle value suits either.

Where does the minimum tank size in the atlas come from?

It is the floor for long-term keeping, not for short-term survival, and it is set by adult size (not the size of the fish you buy), the swimming level it uses and its territorial needs. So a 3cm juvenile of a species that reaches 30cm has to be planned for at 30cm from the start.

What does the compatibility check actually compare?

Five kinds of fact: whether the temperature, pH, hardness (freshwater) or specific gravity (marine) ranges overlap, the ratio between the two adult sizes, each species temperament rating, same-species or same-genus territorial conflict, and any pair already logged in the atlas. One hard conflict settles the verdict as not recommended, and the softer notes cannot pull it back.

Both fish say 26 °C. Why does the check say they cannot share a tank?

A temperature inside both ranges does not mean the two ranges overlap usefully. If one species spans 22 to 26 °C and the other 26 to 30 °C, the overlap is a single point, so any warm spell or cold snap pushes one of them past its limit. That pairing is flagged for caution, not as compatible.

Where does the atlas data come from?

Published aquarium literature, collection records and long-term keeping consensus, expressed as ranges rather than laboratory single-point measurements or one manufacturer specification. Where no consensus exists the field is left empty instead of filled with a number that looks precise, which is why some entries have no carbonate hardness or no mature height.

138 entries

Atlases

This is a reference atlas with ranges you can check, not a list of top ten easy fish. Every entry gives adult size, water parameter ranges, temperament and a minimum tank size, and states them as ranges rather than single values, because a species tolerates different conditions in its native water than it does after several generations of captive breeding. Before you choose a fish, read three things: adult size sets the tank, the water parameters decide whether it can share your existing water, and temperament decides whether the tank ends up with one fish in it.

The atlas splits into freshwater and marine: pick fish by tank mates and minimum tank size, plants by light and CO2, corals by light, flow and aggression, and match a sick fish to a treatment by its symptoms.

Units: temperature in °C, hardness in German degrees (dGH / dKH), marine salinity as specific gravity at 25 °C, volume in litres. These are the values recorded for the species and do not follow the site unit switcher.

Ref.Atlas and what it answersEntries

How to use it

Step 1Start with the water typeFreshwater and marine salinity, hardness and pH are not interchangeable. Decide which water you are keeping first, then browse that section only.
Step 2Filter down to three to five candidatesFilter on temperament, difficulty, minimum tank size or light level. Ask who fits in your tank before you ask which one looks best.
Step 3Open an entry and compare the rangesCheck each range against the fish already in the tank. The narrower the shared window, the less margin you have on temperature and water chemistry.
Step 4Confirm with the compatibility checkerPick two species and read the verdict with its reasons. It does not choose for you; it only pulls hard conflicts into the open.

Frequently asked questions

Can freshwater and saltwater fish live in the same tank?: No. A freshwater fish has body fluids saltier than its water, so water keeps entering through the gills and the kidneys work to expel it; in seawater that reverses and the fish loses water and dies within hours. A marine fish does the opposite and swells in fresh water. The two salinity requirements point in opposite directions, and no middle value suits either.
Where does the minimum tank size in the atlas come from?: It is the floor for long-term keeping, not for short-term survival, and it is set by adult size (not the size of the fish you buy), the swimming level it uses and its territorial needs. So a 3cm juvenile of a species that reaches 30cm has to be planned for at 30cm from the start.
What does the compatibility check actually compare?: Five kinds of fact: whether the temperature, pH, hardness (freshwater) or specific gravity (marine) ranges overlap, the ratio between the two adult sizes, each species temperament rating, same-species or same-genus territorial conflict, and any pair already logged in the atlas. One hard conflict settles the verdict as not recommended, and the softer notes cannot pull it back.
Both fish say 26 °C. Why does the check say they cannot share a tank?: A temperature inside both ranges does not mean the two ranges overlap usefully. If one species spans 22 to 26 °C and the other 26 to 30 °C, the overlap is a single point, so any warm spell or cold snap pushes one of them past its limit. That pairing is flagged for caution, not as compatible.
Where does the atlas data come from?: Published aquarium literature, collection records and long-term keeping consensus, expressed as ranges rather than laboratory single-point measurements or one manufacturer specification. Where no consensus exists the field is left empty instead of filled with a number that looks precise, which is why some entries have no carbonate hardness or no mature height.