This tank was Reef of the Month in January 2018. Pretty spectacular, eh?

Photo from the Reef2Reef archives courtesy of forum member robert s. b. ©2018, All Rights Reserved.

At R2R, we suspect there are plenty of freshwater aquarists contemplating a change to saltwater. There are likely hobbyists who are lurking but are reluctant to ask freshwater questions. The R2R forum, however, is a warm and friendly place. You should feel free to ask anything.

There are many questions we get in the forums over and over again. This is fine. If you are a beginner, please ask questions. There are no stupid questions. So, we thought some FAQs might be helpful to beginners.

This document is by no means an exhaustive treatment of any one subject. I will offer answers that are short, sweet, quick and dirty. I will include links and references at the end so you can read more on the subjects.

Below are answers to some common questions that revolve around the moving from freshwater to saltwater subject. Here we go:

An example of a beautiful freshwater planted tank. This photo from 2013 is entitled "Planted Tank Update."

Photo is courtesy of Geek2Nurse from Flickr via Creative Commons License 2.0. ©2018, All Rights Reserved.

Can I convert my freshwater tank to saltwater?

Sure. Why not?

You’ll need to empty it, rinse it out and dry it first. If it were me, I would sterilize it first, but you don't have to. You also need to consider if you have ever medicated your freshwater display tank because you don't want any traces of any medicines like copper when you are setting up for saltwater.

The biggest difference between freshwater and saltwater tanks—when we’re talking about the tank itself—is the use of a sump. You can certainly have just one display (saltwater) tank. However, most reef aquarists use a sump (or another tank) to house the equipment separate from the main tank. This is especially true for larger display tanks, greater than 40 gallons. If you want to use a sump, you may want to drill the display tank to help with the plumbing.

Would any of my freshwater beneficial bacteria be of use in a saltwater tank?


Some of the bacteria from your freshwater tank may survive in the saltwater tank, but you will mostly grow new strains that are specific to saltwater.

Can I use any of the rock I had in my freshwater tank?


With saltwater tanks, we are always concerned whether anything bad is leaching into the water. You could use some rock that is calcium-based or that is inert in saltwater. You don't want unknown metals dissolving into your tank water. Using live rock or other calcium-based rock designed for use in a saltwater aquarium will be much more porous and will give you much more surface area for growing bacteria that will become part of your biological filtration.

Can I use the substrate that I had in my freshwater tank?

Maybe, but it’s unlikely.

For a reef aquarium, most people have one of the following as their substrate:

A) bare bottom
B) a calcium-based sand
C) a coarser crushed coral

Depending on what you hope to live in your tank, some people experiment with using different materials. Keep in mind that your reef aquarium will most likely have much more flow than your freshwater aquarium did, and the substrate will have to withstand that. Here’s an article on substrates.

Can I use any of the decorations that I had in my freshwater tank?


I wouldn’t recommend using the diver with bubbles or other painted plastic items because there is the risk of unknown chemicals leaching into the water. Most plain plastics are okay, says our resident chemist, Dr. Randy Holmes-Farley.

Reef purists will make fun of you if you have a bunch of non-reef-related artifacts in your reef aquarium. But if it is not hurting your water quality, lose your mind. There are no reef police. What is most important is that it is pleasing to you.

Can any of my livestock go from freshwater to saltwater?


Fish that can only survive within a narrow range of water conditions (and salinity) are called stenohaline. Goldfish, for example, can only live in freshwater. Fish that can survive in a wide range of salinities are called euryhaline. There are plenty of euryhaline fish, but most of them are not suitable for a home aquarium (think bull shark).

Mollies and guppies of the genus, Poecilia, can be moved from a freshwater environment to saltwater if you acclimate them slowly. Some reef aquarists just toss a few into their saltwater tank as feeder fish for their hungry saltwater fish and the feeder fish will not last long. It is probably not a lot of fun for the feeder fish, but I don’t want to debate philosophy here.

Guppies and mollies can live happily in a saltwater tank, and they will breed and bear live young which could potentially feed other tankmates if that interests you.

The question always arises, why would you want to put freshwater fish in a saltwater tank? Well, mollies and guppies are hardy, cheap, they reproduce easily, and many are actually native to brackish or saltwater environments. Saltwater fish are arguably more beautiful, with bright vibrant colors, but they tend to be more expensive.

Nerite snails may be able to acclimate to saltwater. There are species that are used to brackish and saltwater conditions as well as freshwater varieties. The kind you are buying from your local fish store (LFS) may not be obvious. LFS are notorious for labeling snails incorrectly.

As far as plants go, it is possible, but unlikely. Luckily there are several species of macroalgae readily available for saltwater. I would not go to the trouble of trying to convert freshwater plants to saltwater. Nonetheless, you are welcome to try! If you want to try it, select those that thrive in brackish environments and do document it and tell us all about it.

A Balloon Molly, Poecilia sphenops or a Poecilia latipinna hybrid depending on who you ask, bred to have a deformed spine and peculiar shape.


Photo is a royalty-free image from Pixabay. ©2018, All Rights Reserved.

Can I use my tap water like I do with freshwater aquariums?

Probably not.

People do it. A reef aquarium, though, requires narrow parameters that have to be kept stable. It is better to use RO/DI water with zero total dissolved solids (TDS) and just add your aquarium salt. That is the only way to be sure your water has the right chemistry at the beginning. Distilled water can be used too, and it may be smart to keep some on hand for emergencies. Spring water should not be used. It is basically good tap water, but not good enough. And by the way, no, you can't drink your RO/DI water because the resin used in the deionization step is not designed to be food safe.

Remember, no matter what your tap water tests for at any one time, tap water chemistry will fluctuate with the seasons. You would always be aiming at a moving target. I know that this was true in Calgary, Alberta, Canada, where I used to live--there were big swings in phosphate levels throughout the year.

Will I be able to keep and use any of my equipment that I used with my freshwater tank?


Reef aquarists do not typically use the hang-on-back (HOB) filters that freshwater enthusiasts use except on quarantine tanks, breeder/fry tanks, or very small display tanks. It depends on what your goals are with saltwater.

Saltwater tanks most often rely on a protein skimmer instead of a mechanical filter. Sometimes a mechanical filter is still used, but generally reefers rely on filter socks or floss to help clean the water. The live rock in your reef tank provides most of your crucial biological filtration.

(Whatever you use for mechanical filtration, be prepared to clean it more often than you did for freshwater. Junk or detritus that's sitting in a filter becomes what's called a nutrient sink. As the detritus dissolves, it pollutes your saltwater. So, you can't leave mechanical filters unattended.)

You do not typically find air pumps and air stones in saltwater tanks. I would not throw them away, though. You might find a use for them at some point, for example, if you were treating a sick fish (in a separate tank or bath) with formalin, it’s important to aerate the water.

You might be able to use your heater, thermometer, and a few other odds and ends.
Pumps and tubing are usually universal.

I never tested anything in my freshwater tank. Do I have to test water chemistry in my reef tank?


We want you to be successful with your saltwater tank, whether you are a beginner, intermediate reefer or advanced in your experience. Testing parameters is especially important in the early stages of a reef aquarium. The system is not yet mature and testing helps the reefer understand what is happening and where problems may arise.

There are lots of different methods for testing different things that are more or less precise and accurate. You will have to decide what types of test kits or instruments you want to use and what your wallet allows.

Often, over time, a reefer will test less often. As the tank and the hobbyist grow together, there comes a point when the tank seems to “tell” the reefer when there is a problem and careful regular observation of your tank will alert you to a problem immediately.

Will having freshwater experience help me with saltwater?


It depends on who you ask. Many successful reef aquarists have never had a freshwater tank. But there are also many who migrated from freshwater to saltwater.

I, personally, think it's an advantage to have some freshwater experience. At least you understand what changing water means and have at least some vague notions about water chemistry. You might already have some meds, some nets, and assorted other paraphernalia.

Having any kind of aquarium is a commitment, and I'm always nervous when I read about someone with no aquarium experience setting up a giant system with a 200 gallon display tank. But I recognize that I am in the minority to feel as I do. Having freshwater experience is not a requirement to be successful.

Is a saltwater aquarium more expensive than a freshwater aquarium?


While it is possible to do SW on a tight budget, by spacing out purchases, waiting for sales, buying used and doing DIY, there is more of an up-front cost, for example, buying an RO/DI unit for your house. More equipment is usually needed although there are very low-tech SW tanks. And livestock is usually more expensive. And you need aquarium salt every month.

But don't let that deter you. We have plenty of folks on the forum who are starving young students on very tight budgets running terrific tanks.

We hope this little FAQ is useful for the beginners moving from freshwater to saltwater. Join the forum, and let us help you.
A special thanks is due today to the following individual:

Forum member and Reef Squad member NY_Caveman, who took time away from his Christmas dinner to help me with this article. Members of the Reef Squad are especially knowledgeable and patrol the forums to help other aquarists.



We encourage all our readers to join the Reef2Reef forum. It’s easy to register, free, and reefkeeping is much easier and more fun in a community of fellow aquarists. We pride ourselves on a warm and family-friendly forum where everyone is welcome. You will also find lots of contests and giveaways with our sponsors.

Note from the Editor:

Reef2Reef accepts manuscript submissions from outside writers. If you’re interested in writing for R2R, join the forum, and private message me.

Author Profile: Cynthia White

Cynthia received her BA in English from NYU. She has been a freelance writer and editor for over 20 years. Now she is a writer and editor on staff at R2R, where her forum nickname is Seawitch.

For 15 years, she kept a dozen freshwater tanks, bred cichlids--Cyphotilapia frontosa--and sold them to pet stores in Calgary. Finally, after years of study, she has come to saltwater side. She lives in British Columbia, Canada, with her husband and three special-needs dogs, a five-minute walk from the Georgia Strait, the Pacific Ocean between Vancouver Island and mainland British Columbia, where the water temperature ranges from about 8 degrees C (46F) in the winter to 15 degrees C (60F) in the summer. Bring your dry suit. And some hot coffee.