A valid question is what fishes are found in each category of water chemistry. One shouldn’t worry much about keeping a perfect pH or hardness (KH and GH) in aquariums. Most fish can adapt to anything from a 6.5 to an 8.5 pH and any KH and GH. I’ve mixed fish from radically different native water chemistries many times without a problem over a span of fifty years and hundreds of aquariums, as have many others in the hobby.
But if you want to be a perfectionist and keep fish grouped by water chemistry parameters the following will be useful (* indicates a fish which grows very large or is aggressive):
1a, Bacteria Free Soft Acid Water
Delicate fish from the rivers of South America and Southeast Asia which do best in water that is very bacteria free in an aquarium which has been established for more than four months:
- Neon Tetras
- Cardinal Tetras
- Black Neon Tetras
- Glowlight Tetras
- Rummy Nose Tetras
- Hatchetfish
- Ram Cichlids
- Apistogramma Cichlids
- Uaru
- Discus
- Oscar*
- Flying Foxes
- Gouramis
- Rasboras
- Glass catfish
- Neocaridina Shrimp
- Red Tailed Black Shark*
- Rainbow Shark*
- Loaches*
* These are fish which get very big
This is the only chemistry which seems to be touchy. And it has to do not with chemistry but with bacteria. These fish need an aquarium which has a large filter with good media which has been operating at least four months without cleaning. One can be successful with clean, bacteria free water that is over filtered and/or UV treated. These fish are not a problem in hard water with a higher pH of even 8.5 if the water is crystal clear and free of bacteria.
Note that one fish in this group needs to be avoided at the current time. Most dwarf gouramis have a virus disease which kills them at an age of eight months to a year. So one should avoid buying these beautiful fish till breeders solve the problem.
1b, Soft Acid Water
Fish from the rivers of South America where the water is clean, slightly acid (pH 6.0 to 7.0) and somewhat soft. These fish are, in theory, best kept with 1a fish:
- Corydora
- Pencil Fish
- Headstanders
- Leporinus
- Otocinclus
- Most Tetras
- Angelfish
- Acara
- Severum
- Pacu*
- Piranha*
- Knifefish*
*Fish which get very large
2a, Alkaline Water
Fish (largely “African Cichlids”) which are from clean, alkaline (pH of 7.5 to 8.5), moderately hard rift lakes in Africa:
- Aulonocara (Peacocks)*
- Haplichromes (Utaka)*
- Mbuna*
- Lake Victoria Cichlids*
- Synodontis Njassae Catfish
* Aggressive fish
2b, Very Alkaline Water
Fish (largely “African Cichlids”) which are from clean, very alkaline (8.2 to 9.0 pH) very hard water of Lake Tanganyika in Africa. These fish are, in theory, best kept with 2a fish:
- Shell dweller cichlid
- Lyretail Cichlid
- Julie (Julidochromis)
- Blue Neon Cichlid
- Tropheus *
- Lemon Leleupi Cichlid*
- Frontosa*
- Synodontis petricola catfish
- Synodontis multipunctatus
*Fish which can get aggressive
3.Brackish Water
Fish from clean, slightly alkaline (pH 7.5 to 8.0), moderately salty water (brackish):
- Scat
- Mono
- Figure 8 and Green Spotted Puffers
- Archer Fish
- Chromide
- Columbian Sharks
- Bumblebee Gobies
These fish require some salt in their water, a dGH >8 and a dKH >8. If they don’t get that their life will be shortened.
4a, Moderation in the Water
Riverine fish from Africa, Australia, Asia or Central America where the water has a pH of 7.0 to 8.0, is clean and slightly to moderately hard:
- Indian Glassfish
- Congo Tetra
- Angelicus Catfish
- Upside-down Catfish
- Rainbowfish
- Kribensis
- Convict
- Jewel Cichlid*
- Firemouth*
- Redhead Cichlid (Vieja)*
- Red Devil*
- Salvini*
- Jack Dempsey*
*Fish which can get either very aggressive, large or both
4b, “Beginner Fish”
And finally, there are those fish that can live in just about any water chemistry (“beginner fish”). This last category is more accepting of poor water conditions than the other categories. But all the fish listed will have less disease and longer lives in better water quality.
- Molly
- Swordtail
- Platy
- Guppy
- Barbs
- Betta
- Silver Dollar
- Various Danios
- Common, comet, fantail, sarasa and shubunkin Goldfish
- Pleco*
- Chinese Algae Eater*
*Aggressive and/or large
Note the guppy is put in here with some reservations. The highly inbred fancy guppies are often very difficult fish. But many beginners are successful with them. Different guppy strains are inbred to different degrees and thus the huge variation in viability.
The numbers reflect four groups of fish that come from the same type of water chemistry. If one is desirous of keeping a community aquarium in “correct” water chemistry, use this as guidelines. Note there appears to be no research or objective evidence of any correlation between lifespan or health of the fish and mixing groups 1, 2, and 4.
Also note that we don’t recommend mixing group 3, brackish water fish, with any other group. A fish like a scat might live for a year in a Malawi tank but it’s not a good idea.
Tetra, barbs, rasboras, danios, loaches and corydoras are generally schooling fish. Schooling fish MUST be in schools of at least six and preferably ten fish. This is not to keep them “happy”. This requirement is because, in an aquarium with other fish species, these fish CAN become very aggressive if not in a school. Groups of two or three fish of species like tiger barbs, serpae tetras, black phantom tetras, zebra danios and most glofish (bettas and sharks being the exceptions) have become killers in SOME aquariums. Sometimes these fish have killed much larger fish like angelfish if they are not in a decent sized school. So a school is a requirement.
Charts About Groups of Fish
Another way of mixing fish is to look at a chart found on the internet. This is one:
This is a very good chart for most aquariums. It says for instance that African cichlids and South American cichlids “can usually coexist” in an aquarium. The choice of words here is excellent. But this chart lists goldfish as being “Not Compatible” with a whole bunch of fish that goldfish can live with quite easily, so take everything here with a grain of salt.
Note that there are many “fish police” on social media which fervently believe that anyone that keeps fish in water which does not closely resemble the water that the fish is native to, is an absolutely horrible person and should be flailed alive or burned at the stake. It is recommended to simply block these commentators.
Here is a link to an article which discusses the lack of importance of pH in the aquarium. Just click on the link:
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