I've just been donated a mature marine tank. I've had a fair few years experience with tropical freshwater fish, but no real experience with marine tanks. What are the major differences, and what new things will I need to look out for, now I have a marine set up?

  • Not a problem, tropical can also refer to some saltwater setups too, so I wanted to clarify. Do you have corals in your tank?
    – Spidercat
    Commented Sep 25, 2014 at 18:15

2 Answers 2


This answer is with the assumption that you are referring to a fish only marine tank, normally called a FOWLR (Fish only with live rock) tank. A coral reef tank requires considerably more skill and husbandry than a FOWLR tank, but biologically speaking, fish tanks all work the same way regardless of the salinity of the water.


The biological differences between the 2 tanks are very little. Both types of aquarium filtration relies on nitrifying bacteria to convert ammonia (from fish waste) into nitrite, then to less toxic nitrate. Freshwater tanks usually use a hang on back, canister, or undergravel filter for the nitrifying bacteria. Marine tanks normally rely on the bacteria colonizing the rock and sand instead of a stand alone filter. In freshwater tanks, nitrate is typically only removed via water change. Marine aquariums do have the benefit of it being reasonably possible to achieve true denitrification which completes the nitrogen cycle. It's extremely difficult, if not impossible, to get denitrification in most freshwater tanks. In marine, denitrification is accomplished through deep sand beds or other anaerobic environments where denitrifying bacteria can establish, even in live rock. It should be noted that this isn't always desired for a number of reasons and isn't always efficient enough for the stocking of a tank, but it's often possible. As with fresh water, any excess nitrate that the tank can't process on its own, must be removed via water change.

Equipment and Maintenance

Husbandry is going to be pretty much the same with the obvious exception being the requirement of saltwater instead of fresh water. Evaporation should be made up with freshwater. Normally reverse osmosis or distilled water is used to prevent adding additional elements and nutrients to the tank which promotes unwanted algae growth. RO or distilled water should be used to mix the saltwater for the same reasons. Some people are able to successfully use treated tap water, but it's significantly more difficult and usually only works with certain setups.

Saltwater equipment often builds up calcium which slows down water movement and can stop pumps completely. Usually a yearly or bi-yearly soak in white vinegar takes care of this.

Saltwater tanks often use a protein skimmer which needs to be regularly emptied when the cup is full.

If a tank uses foam or a filter sock, these should be replaced every few weeks or as needed for them to keep the water clear.


As far as stocking, marine fish are much more expensive than freshwater fish. Some are extremely sensitive and difficult to care for. Marine tanks cannot be stocked to such a high level as fresh water tanks. There are multiple reasons for this, the main being fish aggression is much more of a problem with marine species. In nature, most marine fish establish a territory and will defend it. When this behavior is applied to a small closed environment like a fish tank, even seemingly peaceful or very small fish can become menacing to the point where they kill the other inhabitants in the tank. Overstocking a marine tank will also almost guarantee a high-maintenance mess of algae and lost fish. Lastly, many marine fish are very active and travel many miles per day in the wild. In an overstocked or undersized tank they live a short and stressed life.


Marine tank do pose several risks to fish and to fish keepers that you won't see in most freshwater tanks.

Unlike freshwater tanks where they rarely exist, denitrifying environments like a deep sand bed can pose a serious risk to the tank and even to humans if kept improperly or if disturbed. They can kill everything in a tank or release hydrogen sulfide into the air which is extremely toxic. Luckily it smells very strongly of sulphur so it's normally obvious if it happens.

Working in a marine tank also poses risks such as infection, allergic reactions, and being exposed to poisonous tank inhabitants. Many coral and anemones which can hitchhike into a tank can sting and some are extremely poisonous. Many marine fish are poisonous and not just obvious ones like lionfish. The fish aggression can also extend on fish protecting their territory from people which in the case of some fish or eels can result in some nasty bites. There are a number of invertebrates that can sting someone working in the tank. Always best to use gloves when moving rockwork or generally any time working in the tank.



The main difference between a saltwater tank and a freshwater fish tank that keeps people from getting into keeping saltwater tanks is cost. There's a big difference between losing a $3 freshwater fish and a $30 saltwater fish. The cost of setting up a new saltwater tank can get insanely expensive. Once it's started though, it's not so bad.

Really though, it's all the same dance, there's just a couple more steps you'll want to take with a saltwater tank.


Salinity is the term used for how much salt is in the water.

Really, there's nothing special about this, if you've ever performed a test for you pH levels in a freshwater tank, it's the same sort of deal. You're just going to want to be testing it at the end of each day, as sudden changes in salinity can be extremely damaging to fish and corals.

When you test for salinity you can get as specific as you're willing to spend money on. At the higher end of the price range, you're looking at refractometers and similar tools that can measure the amount of salt in the water based on parts-per-million. At the lower end of the price range, you're looking at hydrometers, which will measure the gravity of salt in the water.

What gravity the salt needs to be is dependant on what you're looking at keeping in your tank. My general rule of thumb is about 1.02 for fish, and 1.026 for reef tanks.

Water Changes

Water changes are going to be the same as freshwater tanks, except that it's going to be extremely important to have the water prepared beforehand. You have to replace the water with water of the same salinity and temperature as the water you're taking out, otherwise the sudden changes could cause your fish and corals too much stress. Think of it like an ammonia spike in a freshwater tank.


Temperatures work the same way as with freshwater tanks, but there are a few things you'll want to keep in mind.

  • Some fish and corals might not have the same preferences for temperatures.
  • Corals are extremely sensitive to temperature changes.
  • The metabolic rate of corals is dependent on the temperatures of the water. Meaning that if the water isn't at their needed temperature, they can't digest their food. This could simply mean that they don't grow, but the worst case scenario is that it kills them.
  • As I mentioned with the water changes, any new water you're adding into the tank has to be the same temperature, otherwise you risk stressing the fish and corals too much.

General temperatures are going to be about 78-80 degrees fahrenheit. Always keep an eye on the temperatures to make sure they're stable.


The only new piece of equipment you'll really see with saltwater tanks are protein skimmers and powerheads.

Protein skimmers are just another form of filtration that's used to take out the dying microorganisms in the water, since there are millions that live and day in the span of a day in the live rock/sand and corals.

Powerheads are just underwater fans that are used to create a current in the tank, and provide circulation. I consider them extremely useful in reef tanks to make sure that all the corals and anemones receive food.

The rest of the equipment is going to be the same, but more expensive than if it was going to be for a freshwater setup.

Tanks Size

There are very few saltwater fish that will be happy in a tank smaller than 30 gallons. Don't be fooled by the size of the fish either, even a 3-inch long wrasse will need a 55 gallon tank to swim around in.

It's possible to keep a 10-20 gallon saltwater tank, but I personally wouldn't recommend it unless you're familiar with them first. Generally, tanks those sizes are used for nano-reefs with one or two clownfish.

Similar to freshwater tanks, the larger the tank, the more stable your water parameters are.


You won't really use the cheap filters you would use with freshwater tanks. For filters, it works best to use either a canister filter, or a sump.


Rather than the gravel, or plain sand you would use in a freshwater tank, you're going to use what's referred to as 'live' sand. It's 'live' because there are microorganisms living in it that will help filter and cycle the tank.

Generally you'll want to keep your live-sand at a 2-3 inch depth, unless you have animals that dig into the sand like gobies, then you'll want another inch for them.


Unfortunately the salt in saltwater isn't the same as table salt. The only way to make the saltwater is either buy the salt, and mix it in with water, or buy concentrated saltwater, that you would dilute to your needed salinity.

Quarantine Tanks

Most saltwater fish are wild-caught, though some of the more popular fish such as clownfish, cardinals, and damsels are starting to be farmed more widely now. Because of this, it's going to be necessary to put any new additions to your tank into a quarantine tank first, where you can observe them for any signs of illnesses and/or parasites.

Even live-rocks and plants should go into the quarantine tank. It's common to get some bristle worms hitchhiking over, and they're okay for your tank, but the last thing you'll want is to find a brittle sea star popping up in a reef tank, or while fairly rare, a bobbit worm.

It's easier to kill a hitchhiker in a quarantine tank, then to try and fish it out of live-rock where there are so many places to hide.

It's also necessary to treat fish outside of the tank, as treatments will also kill the microorganisms that make up live-rock and coral.


Anemones and some fish will require brine shrimp for their diet. Fish like tangs will require seaweed. Not complicated, but more pricey than simple fish flakes.


It goes without saying, don't touch poisonous fish. But, it's not like they're going to chase you down and try to kill you (And in most cases poisonous means an injury similar to a spider bite). I've had my hands in several tanks of poisonous fish and never had a problem. Just use common sense and don't touch them.

I suggest not putting your hand in the tank if you have cuts on it, though I recommend the same for freshwater tanks. It's likely to make the cut infected which is annoying.

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