Freshwater Versus Saltwater Aquariums: What You Need to Know

By Adam Denish, DVM

Perhaps you’ve wandered into the aquarium department of your local pet store and taken a good look at the faces of the fin-tailed. It’s easy to fall for the beautiful colors, the graceful movements and the soothing sounds of the water. If you feel like you’ve been hooked, read on to learn what decisions beginning fish hobbyists need to make when contemplating the details of adding a new aquarium to their home.

Freshwater or Saltwater?

Water covers about 75 percent of the Earth. In the assorted bodies of water, lifeforms have evolved to survive with varying amounts of dissolved salts and minerals, varying temperatures, oxygen levels and depths. When building an aquarium, you aim to recreate the conditions of the environment where the inhabitants have originated. The pet trade has broadly divided the aquarium habitats into freshwater (tropical) or saltwater (marine). A comparison of the components of these habitats will help new fish keepers make an informed decision about how to select an aquarium suitable for their lifestyle.

The Livestock

Both saltwater and freshwater aquariums feature an amazing cast of characters. In addition to fish, there are invertebrates like snails and crabs to make your tank more interesting. Freshwater tanks can include plants while saltwater tanks can include corals and anemones.

A beginning aquarist might opt for a community freshwater tank filled with guppies, mollies and tetras. It’s a good idea to add scavengers, like snails and cory cats, to a freshwater tank to help manage the build-up of algae. Alternatively, a tank of a single species such as fancy goldfish, discus, or angelfish makes a beautiful aquarium and allows the keeper to become very knowledgeable about the habits of their favorite fish. Freshwater fish can reproduce easily if the water quality, temperature and surface space exists so be prepared with a nursery tank. Aquascaping with live freshwater plants can add additional interest and serve as a wonderful hiding place and food source for the fish.

Saltwater fish are brightly colored and have fascinating habits. The watchman goby occupies a small cave-sized opening at the bottom of the tank, guarding its territory around the clock. Clownfish have symbiotic relationships with certain species of anemones. Pufferfish are friendly to their owners, often swimming to the front of the tank and “begging” for attention. Owners of saltwater aquariums undoubtedly grow attached to their fish and take pleasure in watching the interactions. Compatibility charts should be consulted when choosing tankmates, as fish come from a variety of seas where the species might not ordinarily meet.

A saltwater hobbyist must make the choice of building a fish-only tank or a reef system. While the saltwater fish are fascinating on their own, corals bring a new level to the hobby. Choosing a reef aquarium will determine the type of lighting required and add some additional water quality maintenance. Not all fish are compatible with corals so be sure to do your homework when choosing tank inhabitants.

The Water

The fish in freshwater aquariums have their origins in streams, ponds, rivers and lakes. The water for these fish should be dechlorinated, which can be accomplished by adding a dechlorinating agent to tap water. The water should also have movement to increase the amount of oxygen which can be achieved by using an air pump. Aquarists routinely monitor the temperature of the water to be sure it is stable and also test the levels of ammonia and pH to ensure that waste is properly removed by the filter. Always research the fish you purchase for any additional requirements for the water, as some fish (like koi) swim in colder water or prefer shall water depths (like archer fish, which hunt for insects).

Salt for marine aquariums is available to mix with home tap water to make a ratio comparable to seawater. A hydrometer measures the specific gravity of the water, indicating how salty it is. Some invertebrates, like corals and anemones, require specific additives like calcium and iodine to a saltwater aquarium. Similar to a freshwater set-up, monitors for temperature, ammonia levels and pH are needed.   

The Equipment

A tank, preferably with a hood, substrate for the bottom (like gravel or sand) and a filter that can process the water five to ten times per hour are required for both freshwater and saltwater aquariums.

With regards to the size of your tank, your level of interest can dictate the size of your freshwater tank. From housing a single betta fish in a one- gallon tank to a 350-gallon show tank, there are plenty of options. You will also need a cover above the tank to prevent rapid evaporation. If you choose to house plants or you would like to see the fish at night, you will need to add a light. An air pump to add oxygen and a heater/thermometer to monitor the temperature are also required with freshwater tanks.

A saltwater tank should be at least 30 gallons for best results. Small fluctuations in water quality are amplified in smaller tanks, making them difficult for housing a marine system. In addition to the components for freshwater aquariums, most hobbyists recommend a protein skimmer for saltwater tanks to remove organic wastes. The protein skimmer works in conjunction with the filter and will need to be emptied and cleaned about once per week, depending on the size aquarium and number of inhabitants.

Corals in saltwater tanks have special lighting requirements with regard to the intensity of the light and the number of hours of lighting. There are a variety of lighting options, some with automated light cycles to mimic natural sunlight. A substrate that many hobbyists include in their saltwater tanks is live rock, a porous rock that is inhabited by microscopic organisms including algae and bacteria that will benefit your tank. Rock is sold per pound and can be used as a base for corals and anemones.

The Costs

The cost of freshwater fauna can be very reasonable, with varieties of common fish selling for a little as five dollars. Some of the larger and more ornate fish, like plecos and koi, can be several hundred dollars and are recommended for more experienced aquarists. Building a freshwater aquarium does not have to cost a lot of money. You can start modestly with a ten- gallon tank that is often packaged with a hood, filter and gravel for under 50 dollars. Maintaining a freshwater aquarium does not have to be very time consuming, as most freshwater fish are relatively hardy and forgiving of the mistakes made by new tank owners. With a small investment, a newcomer can build on to the tank as their interest in the hobby grows.

Marine aquariums, however, are an investment in money and time. More equipment is needed for a marine set-up and it is significantly pricier (hundreds to thousands of dollars). Saltwater livestock is considerably higher in cost than freshwater. While there are some fish priced below 20 dollars, most saltwater fish are priced at 50 dollars and up. If owning a saltwater aquarium is something that you are contemplating, be sure to do extensive research and find an experienced hobbyist who is willing to offer advice while you get your feet wet.