In November of 2018, we published an article on some of the initial questions that aquarists ask before starting a saltwater tank. In case you're one of those people still hesitating about whether you want to have a reef tank, here we're going to try to explain some of the major differences between freshwater and saltwater aquariums.

A 93-gallon cube reef tank.
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This photo is from the Reef2Reef archives, courtesy of @vetteguy53081 ©2019, All Rights Reserved.

Equipment

This is a list that appeared in an article about what equipment you need to start a reef tank. This list below was for a small, 40-gallon or less tank without a sump.

NEED FOR SMALL REEF TANK

1) Tank
2) Heater
3) Thermometer
4) Artificial salt or access to natural sea water
5) Hydrometer or refractometer
6) 1 submersible powerhead
7) Live rock
8) Some kind of mechanical filtration
9) Lighting
10) Water
11) a GFCI (ground fault circuit interrupter) outlet

NICE TO HAVE

12) An automatic top off unit (ATO)
13) Protein skimmer
14) Various medications for incoming fish
15) Heater controller that will shut it off if it overheats.
16) RO/DI water
17) Spare heater
18) Spare powerhead
19) Extra tank for quarantine or if a fish gets sick or injured.
20) Test kits
21) Good quality lighting
22) Timer for the lighting
23) Nets for fish if you plan to have any fish
24) Siphon hose
25) Grounding probe
26) Sand
27) If you plan on having hard coral, then eventually you'll need a method for dosing extra calcium

A 187-gallon reef tank belonging to a staff member.
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This photo is from the Reef2Reef archives, courtesy of @Brew12 ©2019, All Rights Reserved.

Now I'll take the same list and put in green what you need if the tank is freshwater.

NEED FOR FRESHWATER

1) Tank
2) Heater
3) Thermometer

4) Artificial salt or access to natural sea water
5) Hydrometer or refractometer
6) 1 submersible powerhead
7) Live rock
8) Some kind of mechanical filtration
9) Lighting
10) Water
11) a GFCI (ground fault circuit interrupter) outlet


NICE TO HAVE

12) An automatic top off unit (ATO)
13) Protein skimmer
14) Various medications for incoming fish
15) Heater controller that will shut it off if it overheats.

16) RO/DI water
17) Spare heater
18) Spare powerhead
19) Extra tank for quarantine or if a fish gets sick or injured.
20) Test kits

21) Good quality lighting
22) Timer for the lighting
23) Nets for fish if you plan to have any fish
24) Siphon hose
25) Grounding probe

26) Sand
27) If you plan on having hard coral, then eventually you'll need a method for dosing extra calcium

So, it's plain to see that you need more equipment for a saltwater tank, not least of which is the salt.

Let's talk about these things one by one.

1) Yeah, you need a tank. And you can use the same kind of tank for freshwater or saltwater, although if you want a big saltwater system, aquarists often choose a tank that has some modifications that make it easier to plumb into a sump.

2) You definitely need a heater for a saltwater tank unless you have a temperate tank, in which case you might need a chiller.

3) You would need a thermometer whether you have freshwater or saltwater. The thermometer lets you know that your heater is doing its job correctly.

4) and 5) Salt and something for measuring salt concentration is specific to a saltwater tank.

6) The submersible powerhead is also specific to saltwater tanks. Saltwater tanks need more flow than freshwater tanks. More on that later.

7) A reef tank by definition has some live rock in it. That's rock that is colonized with beneficial bacteria. You wouldn't need that in a freshwater tank.

8) A freshwater aquarium needs filtration just like a saltwater aquarium does.

9) Lighting becomes very important depending on what you want to grow in a saltwater tank. Most coral need intense and high quality light. Freshwater tanks typically don't need this unless you have a planted tank.

10) Both types of aquariums need water. The quality of the water is more important for a SW tank. More on this later.

11) A GFCI outlet should be used with any type of aquarium, however, SW conducts electricity better than FW. It's an important safety measure.

An unusual reef tank with lots of macroalgae.
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This photo is from the Reef2Reef archives, courtesy of @Exclusive Corals ©2019, All Rights Reserved.

Now let's look at the big picture.

Water

If you have an aquarium, whether it's FW or SW, the quality of the water is important. The quality of the water is more important for a SW tank, however.

Why is that?

That's because saltwater livestock is used to a very constant and unchanging environment. Freshwater livestock from rivers, for example, may see big fluctuations in the water quality during the different seasons of the year. Dry periods, runoff, and heavy rains may all play a big role in the pH, density of ions, and even temperature of freshwater.

Not so with saltwater. The ocean is so vast, that the water parameters are very stable all year round, and that's what you're trying to recreate in your tank.

That's why freshwater aquarists can often get away with using tap water (provided you remove the chlorine or chloramine) but saltwater aquarists struggle with tap water. We usually recommend that you use RO/DI water for a reef tank or water that has been purified with a reverse osmosis and deionization unit down to zero total dissolved solids before mixing with artificial salt.

Even tap water can fluctuate quite a bit throughout the year with its quality. RO/DI water is always the same. Many or most freshwater livestock, aside from the rarest or fussiest, can thrive with tap water. Saltwater livestock cannot.

Flow

Think of the waves in the ocean. Saltwater livestock is used to a lot of movement in the water. This is how corals feed and also how waste is removed from their polyps.

Most freshwater livestock doesn't need a lot of flow. That's why you need extra powerheads in a reef tank to move water around. The movement at the water's surface also helps with gas exchange. In a freshwater tank, you often use an airstone.

Reef aquarists don't typically use airstones for several reasons, but one of them is the breaking of bubbles on the surface of your saltwater tank adds to what's called "salt creep" or the leftover salt and mess that you find above the water line when water evaporates and leaves the salt behind.

Test Kits

Most reef aquarists test their water a lot, especially in the beginning. The test kits let you know that your water parameters are stable, which is what's critical. Since this is less important in a freshwater tank, many freshwater aquarists test little if they bother to test at all.

I bred and sold cichlids for years and never once tested my water. Not once. I didn't even own any test kits. Now I'm not saying that I was doing what should be done, only that it's possible to run freshwater tanks without testing.

Protein Skimmer

Reef aquarists often have a protein skimmer. The skimmer removes unwanted waste and also is a great way to oxygenate your water. Saltwater holds less oxygen than freshwater, and that's why being sure that you're oxygenating it well is very important with a saltwater tank.

Expense

There's no doubt that having a saltwater tank is more expensive than having a freshwater tank. No doubt. There are several reasons for this.

First, you have more equipment and are using more electricity. You need RO/DI water and then you have the cost of artificial salt.

But saltwater livestock also tends to be more expensive. Fish are more expensive and coral is more expensive. That's because most saltwater fish in the aquarium trade are still wild caught. There are currently very few saltwater fish that are bred for the trade.

Lighting

As mentioned earlier, lighting for corals is critical. The quality or intensity of lighting for most freshwater or saltwater fish is not so critical. If you have a freshwater tank, you probably haven't thought twice about your lighting unless you have a planted tank.

For all these reasons above, we recommend that you think about what you want to put in your reef tank before you start buying equipment. In a saltwater tank, what will thrive in your tank is dependent on your lighting, flow, filtration, and even the size of the tank.

And that is the biggest difference between freshwater and saltwater aquariums. When starting saltwater, if you buy everything first, then figure out what you can put in that tank, you will wind up buying twice.

Different types of saltwater livestock have very specific requirements for lighting, flow, tank mates, size of tank, and filtration. So, think about what you want your tank to look like before buying equipment. Join the forum, ask questions, then go ahead and start buying stuff.

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Author Profile: Cynthia White

Cynthia received her BA in English from NYU a long long time ago. She has been a freelance writer and editor for over 20 years. In 2018, she won the President's Award from the Professional Writers Association of Canada. Now she is a writer and editor on staff at R2R, where her forum nickname is @Seawitch.