Making the Freshwater to Saltwater Switch

Author: Jeff Kurtz

During my first decade as an aquarium hobbyist I kept only freshwater tanks. I found saltwater systems equally alluring but considered them well beyond my capabilities. After all, I’d heard and read time and again that marine tanks demand much more work, require great expertise, and are less forgiving of the occasional hobbyist misstep.

What’s more, over on the saltwater side they talked about all kinds of high-tech gadgetry like protein skimmers, special types of lighting, some weird material called live rock, and specimens costing a week’s wages or more. And all those acronyms and abbreviations! From SG and MH to SPS, LPS, and RO/DI, I worried I’d need some sort of decryption key just to have a conversation with fellow hobbyists.

But then a funny thing happened after I finally took the saltwater plunge some 20-plus years ago: I realized that most of my misgivings were unjustified, and it occurred to me that certain myths related to marine aquarium keeping may be standing in the way of a truly rewarding experience for aquarists.

Marine Myths Debunked

Myth: Saltwater Tanks Are Harder to Maintain Than Freshwater Tanks

Among the more persistent myths surrounding saltwater systems is that on the whole they’re more difficult to maintain than freshwater systems. This is untrue: although a full-blown reef aquarium—for example, one stocked with various small-polyp stony corals (such as Acropora spp.) and clams from the genus Tridacna—is unquestionably more challenging to set up and maintain than a freshwater system housing swordtails and guppies, a high-tech, planted freshwater aquarium can be every bit as challenging as a reef system.

For that matter, freshwater setups that feature more delicate and demanding species, such as discus, can be much more of a challenge than marine systems housing relatively hardy species like damsels or cardinalfish. So no blanket statement regarding the difficulty of saltwater versus freshwater aquarium keeping can be considered accurate

Myth: Saltwater Tanks Are Always Costly

One prevalent myth worth dispelling is that saltwater systems are necessarily dependent on lots of high-tech, costly gadgetry. Now there’s no question that some hobbyists’ tanks are extremely technical and feature all sorts of gear and reactors for purifying the water, automating various aspects of upkeep, supplementing elements such as calcium and alkalinity, and monitoring water parameters. But many of these devices—while helpful—are not essential to achieve success with a saltwater system.

In fact, it’s entirely possible for a thriving marine aquarium to be equipped with little more than a protein skimmer, appropriate lighting, a submersible heater, and submersible pumps for circulation. How complicated you make the system is up to you. Of course, the simpler the system the more you’ll need to rely on good old-fashioned elbow grease to keep its inhabitants in good health.

Aquarium Commonalities

Making the switch from freshwater to saltwater aquarium keeping doesn’t mean starting from scratch. Many of the same principles and techniques that promote success with freshwater aquaria transfer directly (or very nearly so) to marine aquaria. Let’s take a look at some of these commonalities.


That’s right, everything you learned as a freshwater hobbyist about cycling a tank—i.e., establishing healthy populations of beneficial nitrifying bacteria that convert deadly ammonia into nitrite and then into nitrate—applies equally to saltwater systems. The species of bacteria involved may be different, but the process is exactly the same and equally important if you hope to keep marine fish and invertebrates alive.

However, having experience with both freshwater and marine systems, I would argue that cycling a new saltwater aquarium can actually be easier than cycling a new freshwater tank thanks to live rock. These rocks contain everything you need to get the cycle established, including a complement of aerobic nitrifying bacteria, ample surface area to support bacterial colonies, and even a source of ammonia, which is produced through the die-off of some encrusting organisms.

Water Changes

Regular partial water changes—either 10 percent weekly or 20 percent biweekly—to remove dissolved pollutants are a mainstay of marine aquarium maintenance just as they are with freshwater systems, though you’ll have the added step of mixing up the saltwater ahead of time. This is accomplished by aerating and heating it for a day or so and then making sure it matches the established parameters of your system (in terms of temperature, salinity, etc.) before performing the water change.

Acclimation and Quarantine

Gradual acclimation of newly acquired fish to the temperature and water conditions in your system is also a commonality between freshwater and saltwater aquarium keeping. To prevent disease outbreaks, it’s vital to quarantine all new animals in a separate tank for observation and potential treatment prior to introducing them to your display tank. I recommend quarantining new specimens for at least four weeks, which is ample time for any disease to show itself. If treatment is necessary, continue to quarantine for an additional four weeks after symptoms are no longer evident.

Sensible Stocking and Feeding

The perils of overstocking saltwater tanks and/or overfeeding their inhabitants are exactly the same as for freshwater systems. In such circumstances the excessive bioload or high level of dissolved pollutants from decomposing food can all too easily outpace the system’s biological filtration capacity, leading to a deadly ammonia spike.

Regular Water Testing

With any aquarium—freshwater or marine—routine testing of pH, ammonia, and nitrite and nitrate levels is essential. Regularly checking water parameters to make sure they’re in the appropriate ranges allows the hobbyist to detect any undesirable shift early on so that it can be addressed and remedied before the health of the livestock is affected.

Compatibility Concerns


Territorial aggression among livestock is also a commonality, and freshwater hobbyists who dutifully research the typical behavior and temperament of species and use that information to guide their stocking decisions will find that these same laudable habits will serve them well on the saltwater side of the hobby.


However, I would be remiss if I failed to point out that they might also run into a few compatibility issues they hadn’t really encountered before. For example, they might be surprised to discover that keeping marine fish species in conspecific groups is often problematic. While there are certain noteworthy exceptions—Chromis sp. damsels and chalk bass (Serranus tortugarum), for example—many species in the trade will squabble incessantly if more than one is kept to a tank. For freshwater hobbyists accustomed to keeping tetras or barbs in schools, this may take some getting used to.


New marine aquarists trying their hand at reefkeeping will also encounter some novel compatibility issues. In addition to ensuring that the fish in their tanks are compatible, they must also address whether their fish will get along with their corals (i.e., refrain from nipping at or eating them) and whether the corals and other sessile invertebrates will coexist peacefully.

This latter point—the importance of peaceful coral coexistence—may be the most difficult for newly initiated reefkeepers to wrap their heads around, since coral aggression is radically different from the aggression exhibited by fish.

Corals may not be able to bite and chase one another like fish can, but they have all kinds of stealthy, insidious ways to go on the offensive. Depending on the species, they may exude toxic chemicals into the water to kill or inhibit the growth of other corals in the tank (known as allelopathy), sting each other with long sweeper tentacles, extrude mesenterial filaments that actually digest tissues, or rapidly grow or overtop one another to hold their position or claim more real estate on the reef. This simply means that advance research is vital when choosing which coral specimens to keep and in determining where to place them in the tank.

Making the Saltwater Switch

We’ve talked about the principles and techniques that are common to both freshwater and saltwater aquarium keeping, but there are also certain elements unique to the salty side that folks making the switch will need to understand. Let’s start with the most obvious difference: the water.


Unless they have access to a source of clean, natural saltwater, marine aquarists must mix their own using a quality artificial sea salt mix and tap water, which preferably has been purified in some manner (such as through a reverse-osmosis/deionization [RO/DI] unit). The salt mix is added until the desired salinity and specific gravity is reached as measured with either a hydrometer or refractometer.

Natural seawater around tropical coral reefs has a specific gravity (an indirect measure of salinity) of around 1.025, which is in the ballpark of where most reefkeepers maintain their water. Hobbyists with fish-only systems commonly maintain a lower-than-natural specific gravity—typically somewhere between 1.020 and 1.025. Whatever specific gravity/salinity is targeted, it’s important to keep the level stable.

Saltwater is used to fill new aquaria and whenever partial water changes are performed in a system; however, top-offs to compensate for evaporation are done using freshwater. This is because as saltwater evaporates, all the solids dissolved in it are left behind. If you compensate for evaporation with saltwater, your specific gravity will continually climb.

Protein Skimmer

As I alluded to earlier, the marine aquarium hobby is rife with gadgetry, much of which—though potentially helpful—can be considered nonessential for a basic saltwater setup, while other instruments might be more appropriately described as “bells and whistles.” (In this hobby, you can get bells and whistles for your bells and whistles!) But there’s one piece of equipment I simply won’t do without in the saltwater aquarium and that’s the protein skimmer.

This device mixes aquarium water with a large amount of tiny air bubbles in a reaction chamber to remove dissolved organic compounds (DOCs), which minimizes the burden on the biofilter and improves water clarity. In the process, skimming also maximizes the dissolved oxygen level and helps maintain a proper pH level by driving off carbon dioxide.

While there are many variations on protein skimmer designs and the manner in which they’re incorporated in the system (some are placed in a sump below or adjacent to the tank, some hang on the tank itself, and some are free-standing), they all work on essentially the same principles.

As the air bubbles injected into the reaction chamber rise vigorously through the water column, molecules of DOCs adhere to their surfaces and are carried to the top of the chamber. A thick foam builds in there, eventually collapsing into a dark, smelly liquid called skimmate, which accumulates in a collection cup at the top of the unit. The hobbyist then empties and rinses the collection cup, eliminating all that nasty dissolved waste from the system. If there’s one device I would urge every marine aquarist to acquire it’s this one. The protein skimmer is the water quality workhorse of marine aquaria.

Live Rock

I’ve already touched upon live rock relative to the cycling process, but what exactly is it, and what other purposes does it serve? Live rocks are simply chunks of rubble broken off from reefs and then settled to the ocean floor to become colonized with various microscopic and macroscopic marine life. Live rock can also be aquacultured in various ways; for example, terrestrially quarried limestone can be placed on the ocean floor, allowing it to become seeded with organisms.

In addition to nitrifying bacteria, quality live rock also harbors anaerobic denitrifying bacteria that convert nitrate to free nitrogen gas, which can keep nitrates in the tank at a manageable level. Other encrusting organisms might include pink and purple coralline algae, various macroalgae, sponges, fan worms, amphipods and copepods, brittle stars and sea stars, tunicates, anemones, coral polyps, shrimp, crabs, and various other critters, some of which will emerge or hatch out over many weeks or months following placement in your system.

Beyond biological filtration, live rock provides a foundation and attachment sites for corals and other sessile invertebrates in a reef system, hiding places and niches for fish and other motile livestock, and a natural aesthetic that is pleasing to the eye. Plus, many of those encrusting organisms can serve as a healthy, natural food source for fish in the system. In fact, some species, such as mandarinfish (Synchiropus splendidus), tend to fare poorly in aquaria that lack a good supply of live rock, along with the tiny amphipods and copepods (often simply referred to as “pods”) they harbor.

Substrate Material

Most freshwater hobbyists are well acquainted with the use of substrates in aquaria, but they do need to be aware that the substrate materials used in marine systems may be quite different from what they’re accustomed to (with certain exceptions, of course, such as those specializing in African rift lake cichlids).

Calcareous Substrates

Instead of the quartz gravels popular among freshwater hobbyists, the substrates of choice for marine tanks should be calcareous, such as aragonite or crushed coral. I prefer aragonite because it dissolves at a relatively high pH and therefore does a better job of helping to buffer the pH at an appropriate level.

Grain Size

As far as the grain size and depth of the substrate are concerned, they depend largely on your goals with respect to livestock and filtration. For example, if your plan is to keep sand-sifting fish, such as genus Valenciennea sleeper gobies, you would want to use very fine, smooth sand. Conversely, for a burrowing or tunneling fish, like the yellowhead jawfish (Opistognathus aurifrons), a substrate of mixed particle sizes that includes bits of rubble would be appropriate. If your goal is to create a deep sand bed to aid in denitrification, you would want to use a bed of fine sand that is at least 4 inches (10 cm) deep.

Otherwise, if your only concern with respect to the substrate is aesthetics, you can go with whatever grain size you prefer and limit the depth to just a few inches (5 cm) to create a natural seafloor appearance. However, be aware that the larger the grain size of your substrate material, the more it will tend to trap detritus.

Bare-Bottom Reef Tanks

It’s worth noting that bare-bottom reef tanks are growing in popularity. These tanks use means other than substrate to buffer pH and aid in anaerobic bacteria surface area. The benefit of having a bare-bottom reef tank is that the risk of a failing powerhead or a powerful return pump stirring up debris is greatly reduced. If debris is blasted through the water column, it can cause major health problems for many coral species.

Specialized Reef Lighting

Freshwater hobbyists who keep planted tanks will already understand the importance of providing lighting of the proper intensity and spectral quality. However, those who have kept only fish and are switching to saltwater reefkeeping must be aware that most of the corals, anemones, and clams kept in marine aquariums have specialized lighting needs.

The tissues of these invertebrates house symbiotic algae called zooxanthellae, which use sunlight to produce nutrients through the process of photosynthesis and share this food with their invertebrate hosts. If this source of nutrition is absent, photosynthetic invertebrates cannot sustain themselves, so providing proper lighting to support the zooxanthellae is critical.

Supplementing Vital Elements

Last but not least, another novel concept to saltwater aquaria is the need to supplement various elements depleted by corals and invertebrates. Calcium, magnesium, and alkalinity levels all need to be kept in a range specific to your coral species. To accomplish this, daily dosing is often necessary and new saltwater aquarists can decide between manual daily dosing or automated dosing consistent with the “bells and whistles” phenomena of reef aquaria. While reactors exist that can aid in this, most aquarists new to marine aquariums will fare better with three-part dosing.

The Salty Side

Of course, what I’ve touched upon here merely scratches the surface, but if you’re a freshwater aquarist who’s contemplating exploring the salty side, I hope this article has given you some encouragement and food for thought. With the skills you’ve already mastered, saltwater success may be within your grasp. Don’t let a few myths or misunderstandings frighten you away from this fascinating and rewarding aspect of the aquarium hobby.