While marine waters cover more than 70 percent of the earth’s surface, freshwater covers only 1 percent. Surprisingly however, 40 percent of the 28,000 species of fish dwell in fresh waters. This is a very high proportion of species density given the small area that the bodies of freshwater actually covers. This data indicates that there are approximately 1 species of fish for every 15 cubic kilometers of freshwater and 1 species of fish for every 100,000 cubic kilometers of marine waters. Constantly changing environments and ease of geographical separation of small bodies of water in freshwater habitat have resulted in a high degree of diversification of freshwater fish. The constantly changing environments have also forced freshwater fish to become more adaptive to their environment. In comparison, saltwater fish have been able to enjoy a relatively more stable environment in a larger ocean environment. Therefore, freshwater fish are generally more adaptable and hardier than saltwater fish.
Anatomy and Physiology
There are no significant anatomical differences that universally distinguishes freshwater fish and saltwater fish. The difference simply lies in the way that they regulate water and salts in their internal cells. Most freshwater fish and saltwater fish maintain a salt concentration in their blood of approximately 10 parts per thousand (ppt), or 10 grams of dissolved salt per liter of water. Since freshwater fish swim in water with approximately 0.5 ppt, the chloride cells in their gills are designed to pump sodium, calcium and chloride into the fish. On the other hand, since saltwater fish swim in water with approximately 35 ppt, the chloride cells in their gills are designed to pump salt out of the out of the fish. This process of controlling the flow of water across their body is called osmoregulation.
Freshwater Aquarium vs Saltwater Aquarium
When choosing between a freshwater aquarium setup and a saltwater aquarium setup, it is important to realize the difference in level of difficulty and level of care required. As mentioned, freshwater fish are hardier in general due to their adaptive nature. They are accepting of a wider range of water parameters as well. On the other hand, saltwater fish are generally more demanding of specific water parameters and required habitat. While market value of fish can vary depending on the season and distance from the coastal lines, marine fish are much more expensive compared to most freshwater species. Due to the additional equipment involved in a saltwater aquarium and the price tag of the fish itself, a saltwater aquarium setup in general is as twice as expensive as a freshwater aquarium setup.
Due to the difference in cost and the level of care required, most novice aquarists decide to start the hobby with a freshwater aquarium. Just like saltwater fish, there are many colorful and attractive freshwater fish including cardinal tetras, fancy guppies, killifish, bettas, and cichlids, just to name a few. While generally not recommended for novice aquarists, discus fish is another very attractive freshwater fish with a variety of coloration. Even within the scope of freshwater aquariums, there is a great variety of tank setups available. Freshwater tropical community tanks are one of the most popular aquarium setups for both novice and advanced fishkeepers. While it is relatively inexpensive to set up and maintain, one can also appreciate a great variety of fish in a single aquarium. For community tanks however, it is important to understand the requirements of each species in the community in order to ensure compatibility. Since different species have different swimming patterns, combining top level swimmers, middle level swimmers, and bottom dwellers can be an attractive effect in a community tank. Basic components in a freshwater aquarium setup includes filtration, lighting, and heating. For freshwater aquariums without a heating mechanism, coldwater fish are most suitable. Coldwater fish includes goldfish and white cloud mountain minnows, among many other small river species. Another type of freshwater setup are fish bowls. Species of fish that can survive in bowls are very limited. While goldfish are commonly presented in small bowls, this is not very practical. Due to the high amount of waste output of carp species such as goldfish, a fish bowl without a filtration system will struggle to support the fish. Betta fish is a better candidate for a fish bowl since the developed labyrinth allows the fish to breathe atmospheric air. Finally, as a general rule of thumb it is advisable to provide at least 1 gallon of water for 1 inch of freshwater fish.
There is a great abundance of colorful marine fish with striking patterns and body formations. While different species have varying requirements, saltwater aquarium setups can be categorized into three basic types. The most basic marine setup is the fish-only (FO) saltwater aquarium setup. This type of aquarium setup is well suited for novice saltwater fishkeepers due to its simplicity and the relative ease of care. FO aquariums are decorated with coral replicas instead of live rocks. Hardy marine fish such as Damselfish, as well as other tank-bred species, are great candidates for FO aquariums. Another type of marine tank setup is fish-only-with-live-rock (FOWLR) saltwater aquariums. Live rocks, which are pieces of mature coral reefs, allows colonization of a various marine life such as invertebrates and sponges. Since live rocks house an abundance of beneficial bacteria, it aids in filtration and maintaining desirable water parameters. Since live rock require specific acclimation techniques, lighting specifications, and supplements, FOWLR saltwater aquariums require a higher degree of care than FO saltwater aquariums. The most challenging type of aquarium, more so than FOWLR aquariums, are reef aquariums. In reef aquariums, the primary focus is placed on the corals, invertebrates, and anemones. While fish can be present in a reef aquarium, they are considered as an accessory of the entire setup. Reef aquariums require specific water parameters, lighting conditions, and water flow that must be maintained on a very regular basis. Thus, reef aquariums are generally reserved for the most advanced aquarists. Finally, as a general rule of thumb it is advisable to provide at least 5 -10 gallons of water for 1 inch of saltwater fish.
Brackish water aquariums lies between freshwater and saltwater aquariums. Difficulty in maintaining a brackish aquarium depends on the type of fish that is being housed. For novice fishkeepers, hardy fish such as Mollies are recommended for brackish aquariums. Just like a saltwater aquarium, a hydrometer is necessary in order to monitor the salinity in a brackish aquarium. Whenever there is a change in salinity, it should occur gradually in order to allow the fish to properly acclimate.