Freshwater Aquaria Introduction: A Guide to Selecting Fish for Your Freshwater Aquarium

Starting a new aquarium can be exciting, but it can also be overwhelming! There are many decisions that need to be made; tank size, filtration, heating, decor, lighting, and livestock choice all matter in the immediate and long-term health and well-being of your aquarium. 

Determining species compatibility before adding them together is critical, both to save your time and money, and also to ensure the health and safety of your fish. In the beginning, it can be really difficult to know what can and can’t go together, but this article is kicking off a series of guides for you so that you can see various options for different types of freshwater tanks you could bring into your home. Here are our top 3 tips for selecting fish for your aquarium, regardless of the type of tank you end up keeping:

Keep environmental needs in mind while setting up your tank. Each species has different requirements for tank size, water temperature, pH (a measure of how acidic or basic your water is), hardness (the mineral content in your water), and some species are more sensitive to fluctuations in water quality than others. Even if some species are compatible behaviorally, they can be very, very different in their environmental requirements. As an example, most tropical fish found in the hobby will be comfortable at water temperatures of 75-78°F, but some species fall into the other extremes of water temperature. Axolotls are a popular fully-aquatic amphibian in the aquarium hobby, and they are most comfortable in the temperature range of 60-64°F. If you get higher than 70°F, axolotls can begin to show signs of heat stress, and prolonged exposure to temperatures 75°F or above can be fatal. 

Try to keep species together that have similar diets. It may seem like an obvious recommendation, but there are fish that are behaviorally compatible but have very different dietary need. If herbivorous fish are fed a diet of meat protein based pellets or flake foods, there is typically too high of a protein content than their bodies can handle and they can have digestive issues. A great example of this would be Mbuna African Cichlids; they can develop a condition called ‘Malawi Bloat’ when fed too much protein because their digestive system is designed to break down plant matter, and not protein. Alternatively, other carnivorous fish that are fed a plant-based diet won’t be able to digest the plant fibers and won’t be able to get the nutrients they need. If strict herbivores and strict carnivores are kept in an aquarium together, it can be difficult to separate them to feed them their appropriate foods while avoiding the foods that may cause them harm long-term. If you have fish together that need different diets, such as keeping carnivorous bottom-feeders in with other herbivorous fish, “target-feeding” using a pipette is the best way to overcome this obstacle, but it won’t be 100% effective. 

Other critical factors for success are species size and temperament. Even if you have a large fish tank and the species you like have the same water parameter requirements, this doesn’t always mean that they are compatible. Tiger barbs are an excellent example: they are very popular in the hobby because of their larger size (for schooling fish) and bold stripes. However, they have a tendency to be nippy and aggressive with other fish. They will pick on most other peaceful community fish, sometimes even those larger than them, and their aggression tends to be more intense when kept individually or in very small schools. We’ve heard many stories about tiger barbs killing other fish in people’s aquariums, and they had no idea the barbs would be so aggressive! Fish also need to be similar sizes to their tankmates. When large and small fish are kept together there is often a good chance some trouble will ensue. Best case scenario, the little fish are outcompeted for food. Worst case scenario, the smaller fish becomes lunch!