Converting from Freshwater Tank to Saltwater

Converting Freshwater Aquarium To Saltwater

You’ve been amazed by the beauty of the fish and invertebrate, the stunning variety of the species and the subtle grandeur of all the corals and algae that you could only have with a saltwater tank. You now have or have had a freshwater aquarium and have probably wondered what it would take to convert your freshwater tank to a saltwater aquarium?

What Equipment Do You Need?

Of course, you’re wondering…how much new stuff I need to buy? How difficult will it be? Chances are good that you already have much of the equipment you’ll need so the conversion shouldn’t be that difficult anymore.

You probably already have the following:

  • A tank and stand
  • A heater
  • A thermometer
  • A canopy (or aquarium cover)
  • A light
  • Outsider filters/pumps
  • Water conditioner
  • Test kit….still usable if it can measure levels in salt water environments, but replace if more than a year old.

What you’ll need to buy for your saltwater tank:

What size tank?

Let’s talk about aquarium sizes first. We recognize that many tank sizes exist – the smallest nano tanks (typically 5 to 25 gallons), more typical sizes (29 to 75 gallons), larger (100 to 450 gallons), large (500 gallon-plus). We’re going to assume that you don’t have a large aquarium that could double as a jacuzzi in your house – at least, not yet.

So maybe you’re thinking, OK, let’s start realistically. Because the nano tanks are rather touchy and unforgiving of poor water quality, we recommend them only for more experienced aquarists.

Your best choice for a beginning saltwater tank is probably a 55-gallon, largely because it has enough water to allow the aquarist a little more variety and some forgiveness for less-than-ideal water. You can also have success with smaller tanks, such as a 30-gallon.

Fish only or Fish and Coral?

Here’s something else you need to think about: Will you want a fish only with live rock (FOWLR) tank? Will you want a fish-and-coral tank or will you want predominantly corals with a few or no fish? Understand that fish add to the tank’s bioload; they generate wastes that you’ll need to take care of in some way. Corals and invertebrates do not add to the bioload.

This is an important distinction that can help you determine the size of the tank.

Add Water

Next, let’s talk about your water. If you had a freshwater tank and tropical fish and if you used municipal water, you had to buy a de-chlorinator before you added the fish to the tank. If you have well water, you might have used some sort of water softening/metal/ion removing system. If you’ve used de-chlorinator, keep using it. If you have a water softener, keep using it too.

Now, what about the saltwater itself? Since saltwater fish, corals, invertebrates, and algae live their lives in saltwater, you have to replicate their environment. Luckily for us all, synthetic saltwater mixes were first introduced about a quarter-century ago, so we now can have ocean-quality water in moments – literally by just adding water. Remember, you are creating the home for your fish, corals, etc.

Water quality is the single most important aspect of their lives and success. Good water quality not only minimizes stress on the reef inhabitants, but it also minimizes the risk of the effects of stress.

For these and many other reasons, we use Instant Ocean. While other brands are certainly available, we and many of our customers feel confident that the Instant Ocean provides the salt and trace elements in the right proportions that saltwater tanks need.

Instant Ocean provides an assay (chemical analysis) of each lot. Instant Ocean has minimized its variability from batch to batch. It helps assure you of good batch-to-batch water stability.

Measure Salinity

One aspect of saltwater aquariums is different from most freshwater tanks: Salinity. To measure salinity, you will need to buy a hydrometer that measures specific gravity. While different species of fish and invertebrates require different salinities, most aquarists opt for a gravity around 1.022 to 1.024. At this gravity, many species of fish and corals can co-exist in comfort.

Set Up Filtration and Aragonite

Finally, you also have some sort of filtration system – outside filter, canister filter, powerheads, under gravel filters, etc. You also probably have gravel or bottom material. You will probably be able to use your outside and canister filters, but you won’t need the under gravel filters or gravel anymore. Consider giving them to a friend who might like to start with a freshwater tank.

Instead of gravel, you’ll need some sort of material like aragonite. Unlike gravel (which is chemically inert), aragonite provides a source of calcium that helps maintain a pH of 8.2 in your saltwater tank. Aragonite also provides carbonate buffer, strontium, magnesium, molybdenum, and potassium. (For readers unfamiliar with the term “buffer,” a buffer is a chemical that keeps pH at or close to a particular level.)

While there is a discussion on how much aragonite you need, we generally recommend between an inch and three inches per tank, or about one to one-half pounds per gallon. How much is up to you.

Now, let’s talk about filtration. As you know, there are generally three types of filtration – mechanical, biological, and chemical – in a tank system. If your outside filters circulate about 10 times the quantity of water in the tank per hour for a coral tank, that’s adequate. Tanks with fish require higher water flow, about 15 to 20 times the tank volume per hour because fish generate significantly more waste.

Remove Ammonia, Nitrates, Nitrites, and Other Water Impurities

How best to remove the dangerous impurities – remove ammonia, nitrates, nitrites, and other water impurities? The simplest and probably best is this: Weekly or bi-weekly water changes that replace about 10 to 20 percent of the tank water.

Another method is to refitting your outside filters with Boyd’s Chemi-pure. An ion exchange resin, Boyd Chemi-pure scavenges (removes) ammonia and nitrates. Since you replace it about once every six months, it’s significantly cheaper than carbon filters. Boyd’s also makes a Chemi-pure that will remove phosphates and silicates, those two primary contributors to brown and green hair algae in a tank.

Add Live Rock

But the real material that makes for a successful saltwater tank? Live rock, that wonderful, formerly ocean material that contributes millions of beneficial bacteria to a saltwater tank. Since live rock plays a major role in converting toxic wastes to non-toxic byproducts, we strongly recommend seasoned live rock for a tank. Plan to buy a pound to a pound and one-half per gallon.


So by now, you should be able to see that converting from a freshwater to a saltwater aquarium is neither difficult or expensive. Just be sure to regularly test your water and acclimate your fish and corals slowly. Over time we hope your tank continues to evolve into a stunning masterpiece! Please feel free to post photos of your new saltwater masterpieces in the comments below.

We will be happy to hear your thoughts

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