Rare types of Betta fish

If you are interested in cultivating a peaceful tank full of multiple species, don't choose these fish.

There are no such thing as bad fish, just owners who have not done proper research before purchasing them. “Community” tanks can be constructed in a variety of ways, it just depends on what kind of community you want. For example, Lake Malawi cichlids, when kept in the appropriate water parameters and in the correct set up, can create an active and interesting social community. This is also true for aggressive fish such as the red-tailed black shark, or shy and sensitive fish like celestial danios. The key is to read up on the conditions that a fish will prefer before buying it.

Realistically, most people want a bright and active tank with many different types of fish in it, and will just walk down the “community” isle of the local fish store picking out favorites. If this is your technique, here is a list of common community choices that are not well suited for a standard community tank – it may be because they require special water conditions in order to survive, they might be extremely sensitive to water quality, they are subjected to inhumane practices or are just generally difficult to keep alive and happy. For a list of fish that grow too big for a standard tank, please see “Top 10 worst tank busters”.
The 10 worst community fish are:

10. Plecos

Plecostomus, often called “plecos, ” are a very popular fish and they make a wonderful addition to the freshwater aquarium. This may be true, but there are still some major drawbacks to keeping them. Common varieties can grow to be a foot long and they produce massive amounts of waste - even smaller ones can still manufacture disproportionate levels of waste. You can basically double the bio-load of your community tank by adding one of these fish. In addition to that, some plecos don’t consume very much algae and may preferentially shred plants and driftwood. You should also keep in mind that while juvenile plecos are considered a community species, they often become aggressive as they grow.
Female L144 Pleco by Anthony Corda via Wikimedia Commons
9. Glass catfish
These little guys look like they could add visual interest to any community aquarium because of their uncommon appearance. Their bodies are transparent so that you can see right through their skin and muscle to their bones – quite a striking sight! This would make for a fascinating addition to your aquarium – that is, if you ever saw them. The are terribly shy, and require medium sized shoals (7-10 fish) in order to be even remotely comfortable. Even then, however, the entire group will probably spend most of their time hiding behind your decorations, plants and filter inlet. Unless you have a refined appreciation for subtlety in your aquarium, you should look for a bolder substitute. Additionally, they are very fragile fish, and can perish easily both from instable water conditions and from bodily injury.
8. “Fancy” guppies
Fancy guppies are extremely appealing for a community tank because they are friendly, small and come in a range of brilliant colors and striking patters. Unfortunately, most guppies can barely survive the trip home from the pet store. Decades of selective breeding has resulted in stocks that are inbred and weak, and males seem to fair particularly poorly. There are alternatives however – “feeder” guppies can actually be quite pretty, and if you allow them to breed for several generations visually appealing traits often intensify. If you choose to bring home feeder fish however, quarantine them before introducing them to your community tank as they may harbor diseases.
7. Angelfish
While these fish can make graceful and elegant centerpieces in a tank, they are not well suited for a community atmosphere. Angelfish, a species of cichlid, are aggressive to their own species and need to be kept in breeding pairs or in large groups. Keeping them in small groups will allow for the strongest to bully the weakest fish, sometimes to death. As they get larger, they may pick off smaller fish for food. They also prefer specialized conditions in order to thrive. They are sensitive to water quality, and need a slightly acidic pH (6.5-7), low hardness and a warm temperature (~80F). Additionally, the long fins of these fish make the irresistible to common community species such as the black skirt tetra who will chase and nip them which can increase their risk for bacterial and fungal infections.
6. Discus
These fish, like angel fish, are beloved for their bright colors and interesting personalities. Also like angelfish, they require special water parameters – soft, stable, slow moving and warm water. They are shy around other species of fish and can be easily outcompeted for food. They actually will do best when kept in species-specific tanks with lots of space to swim around in and a calm environment in which to interact with conspecifics.
Symphosodon discus by Doronenko via Wikimedia Commons

5. Dye injected fish

If you’ve paid a visit to your local pet store recently you may have come across neon or dye-injected fish. These fish look like normal tetras, danios or glassfish but with bright, unnatural colors. The practice of injecting dye into fish is considered inhumane and can greatly reduce the lifespan of the fish due to stress. The popular trend Glofish, however, involves genetic alteration rather than the injection of dyes. Think carefully before using any of these fish in your tank because they may not be as hardy as traditional community fish and could end up spreading disease.
4. Dwarf gouramis
Gouramis are elegant, beautiful fish that when kept with proper tank mates can be an excellent component of a community tank. However, gouramis of the dwarf variety, usually colored powder blue, royal blue or red, can cause huge problems. These little Napoleons can be quite aggressive, and males often bully each other (as well as everyone else). Additionally, dwarf gouramis frequently carry diseases that can quickly kill other fish. If you are looking for an alternative, keep pearl or honey gouramis (in groups, with 2 females to every male) and be sure to avoid the chocolate gouramis (too sensitive) and larger species like kissing gouramis (too aggressive).
3. Goldfish
Though they are a common “beginner” fish that frequently end up in bowls, tiny tanks or just mixed in with other fish, goldfish are not suited for community tanks for a variety of reasons. This is a group of fish that have been neglected and misunderstood for decades. They require cold water with no heater and their high oxygen requirements make them incompatible with most other fish (and make the standard goldfish bowl completely out of the question!) They also grow quite large, so if you would like to keep goldfish, place them in an outdoor pond.
2. Freshwater eels
Eels are inappropriate for community aquariums for a variety of reasons. Many species grow to be over a foot long (some exceed several feet) and they are difficult to care for. Eels are esteemed escape artists – they love to climb into filters or slip up and out onto the floor. Many species are also sensitive to water quality and they require special substrates that will prevent them from scraping and damaging their skin. Some species are also very shy and easily frightened, preferring to burrow into the sand to stay out of sight. Additionally, they can be difficult to feed, many species prefer live food and will eat (or try to eat) their tank mates.