Saltwater Chemistry

Saltwater chemistry is more complex than freshwater. Nutrients such as nitrate and phosphate play a much bigger role in algae control. And if you are keeping stony corals then calcium, alkalinity, and magnesium become very important as well.

This is not the most extensive examination of saltwater chemistry available. Like so many topics in this hobby, entire books have been written on what just this article attempts to cover. You may have a very successful reef with the information listed below, or you may choose to go into much more detail. This will at least give you a good foundation to build on.

Do not simply start dosing things just because you think you need it. You need to test and only dose as needed. Don’t just start dosing iodine because you bought Xenia or a shrimp. Most likely you will never have to even think about dosing iodine, but if you have reason to think you should then you should buy a test kit and know for sure. Any parameter can be too high, dosing blindly is asking for serious problems, worse than if you didn’t dose at all.

If you use a high quality salt mix and keep up weekly 10% water changes then most of the chemistry will stay exactly where it should be. In a reef, you may need to dose certain things, but do so only as needed when you actually need to, not just because someone online or working at an aquarium store said you should start adding something.

If you do need to start dosing anything, the easiest way is to use liquid additives. With almost all liquid additives, you simply follow the directions on the bottle. If you have a sump then you should definitely add any additives to the sump (this includes RO for topping off and saltwater for water changes). This helps dilute anything before the corals are exposed to it. If you have a very large system or want to be as cost effective as possible then you may decide to start using dry additives. These are more cost effective because you aren’t paying for water. If you use dry additives you will need to dissolve them in RO water first, and then add them. Always follow product instructions, there are always exceptions.

Natural Nutrient Loads

One very important difference between freshwater and saltwater chemistry that is vital to understand is that natural seawater has very little nutrients in it. This is one factor that allows the waters to stay so crystal clear. It is also why agricultural runoff is so damaging to marine systems near rivers.

In a reef, the seawater has very little nutrients. This is why many of the algae that do live there live inside corals, so that they can use the waste from the corals as their source of nitrogen and phosphorous. This also means that marine algaes are well adapted to very low nutrient availability and are very efficient at using what little is there. This is why even the low levels in tap water are more than enough to turn a tank into an algae farm, even with relatively low lighting.

This is why it is important to start with RO/DI water (reverse osmosis/deionization) and to make your saltwater from scratch using a high quality salt.

RO/DI Water

Reverse osmosis (RO) and deionization (DI) are two methods of removing everything from tap water so that you are starting with pure water. There are all sorts of undesirable chemicals in tap water that will cause various problems in a saltwater aquarium. The biggest issues are nutrients such as nitrate and phosphate. Reverse osmosis is the process of removing these chemicals. Not all reverse osmosis machines are equal. RO systems made for drinking water are not as good as those made for aquariums.

There are multiple stages to any RO system and they generally consist of: prefilter (mechanical just to remove particulates), carbon (to remove chlorine, chloramine, and other chemicals), and the actual reverse osmosis membrane. RO systems have one input and two outputs: one output is clean RO water, the other is waste water which is a concentrated version of the tap water containing lots of the bad stuff you don’t want, although this can be great for many house plants.

Deionization uses a special resin media to remove ions (both positive and negative). In aquarium RO systems, the DI always comes after the RO (the clean water from the RO goes to the input for the DI). Some systems include DI as one of their stages, but with others you will have to add it. It is best to have two DI stages so that no matter how exhausted the first is, the water still has to go through a whole second stage. When you replace the first stage, move the second stage to the first and add the new cartridge to the second stage.

Pure water does not conduct electricity. Because of this we can measure the purity of water by conductivity, which measures the total dissolved solids (TDS). A TDS of 0 means the water is not conducting any electricity. Tap water can be anywhere from around 150 to well over 400. Drinking water RO systems typically get the TDS down to 70 or so. Good RO/DI systems for aquariums can get it down to 0, at least when all the stages are new. The single digits are fine, as the TDS goes up you may need to replace certain stages in the RO unit in order to get the TDS back down again. TDS is measured with a small, easy to use meter that you can buy for $10 or so.


Salinity is the measure of the concentration of salt in the water. The best way to measure this is with a refractometer. Hydrometers are cheaper, but they are a pain to use and not nearly as accurate. High quality refractometers can be bought online or at your local fish store for as little as $30.

When we talk about the measure of salinity we actually measure the specific gravity (the density). Saltwater is heavier than freshwater, and the more salt there is in the water the heavier the same volume of water is. This difference in density effects how light passes through the water, which is how a refractometer works.

Natural seawater has a specific gravity (commonly referred to as salinity) of 1.026, stated as one point oh two six, ten twenty six, or simply twenty six. If you have any corals or invertebrates then you need to maintain your salinity at 1.025 to 1.026. If you just have fish then you can go lower. Many people report that their fish do better at lower salinities due to the lower osmotic pressure (a smaller difference between the internal salinity of the fish’s tissues and the outside salinity in the water). Most likely you will not have a strictly fish system, so this should not matter.

A high quality salt mix will provide everything your aquarium will need, from sodium and chlorine all the way down to trace elements that you haven’t heard of since your chemistry class in high school. Some mixes are designed for reef tanks and will therefore have higher amounts of chemicals that are used up quickly in reef systems such as alkalinity, calcium, and magnesium.

Which salt mix you use may seem like a huge decision, but fortunately, most salt mixes are actually pretty good. The most popular is Instant Ocean. This is one of few cases where the most popular isn’t just some cheap, low-quality product that should be avoided. Instant Ocean is good, at least until you start getting stony corals, at which point you may upgrade to a reef salt. Reef Crystals is the reef version of Instant Ocean, and is a great salt mix for reef tanks. There are lots of others though, most of which are very good. You will find that everyone has an opinion, and some people just say, “Use whatever works best for your tank,” as if you are supposed to run trials of every salt mix on the market in controlled experiments until you get it figured out. See what your local fish store actually stocks and prefers, and ask them why. Sometimes it is just what they can get the cheapest, but any decent reef store will be using a good salt mix. And the store actually having it on their shelves may be reason enough to choose that brand over one of the others.

One common mistake that many people new to saltwater make is topping off with saltwater. When water evaporates from the tank only the water leaves, the salt stays behind. So if you were to never top off, the salinity would steadily go up over time. If you top off with saltwater this will happen very quickly. You must top off evaporation with RO/DI water. Ideally you should top off once or twice daily. It is easier to set up an automatic top off (ATO) system which uses float valves to sense the water level and pump in RO/DI water from a reservoir to refill the water level back up to the desired level.  By using an ATO you just have to top off the reservoir as needed instead of topping off the tank one to two times daily.


pH is relatively straight forward. Saltwater aquariums should run 8.0-8.4.

As long as you are doing weekly water changes with a high quality salt mix you shouldn’t have problems with pH, at least not until you have enough stony corals that they are taking out so much KH that you really need to start dosing. There are lots of buffers available that increase KH which will increase pH. Test both KH and pH and dose as needed. Be careful, adding too much too fast can be much worse than just leaving it where it was. Start with less than you think you will need and then add more if needed.

Don’t worry too much about pH. If the pH is too low, focus on correcting the KH. If you can keep the KH in its ideal range the pH should be fine. If you are using a soda ash alkalinity additive (sodium carbonate) as opposed to baking soda (sodium bicarbonate) and still can’t get the pH up where you want it, you may want to consider a CO2 scrubber for the air intake of your protein skimmer. There can be enough CO2 in the air to reduce the pH in the aquarium. In some cases, using a CO2 scrubber can increase the pH by 0.2 or more, so it is worth considering.


Nitrate plays an even bigger role in saltwater than it does in freshwater. It is a measure of overall water quality. Even low concentrations can cause major algae problems. In addition, nitrate inhibits calcification by stony corals and clams. Ideally, nitrate is undetectable to very low. Very low levels (up to 5ppm) may be okay in some systems, especially if you have soft corals such as zoanthids and mushrooms.

As in freshwater, nitrate is a matter of nutrients in and nutrients out. Assuming you are using RO/DI water, nutrients are added only by food and taken out by water changes, refugiums, live rock, and protein skimmers. Having really good nutrient export, generally a strongly lit refugium and a strong protein skimmer, allows for slightly heavier feeding which can be very beneficial. If you don’t have the nutrient export ability to handle heavier feeding then you either end up with a polluted tank or livestock that is on a forced diet. I hate to limit the feeding just to keep the tank balanced. If you even think you need to do this then you need to re-examine your refugium, protein skimmer, and water changes.

Protein skimmers remove certain forms of waste before they break down into ammonia and eventually nitrate. By removing these early in that process, you completely avoid the nitrate in the first place.

A refugium is one of the best ways to remove nitrate. The macroalgae in the refugium will suck out nitrate and phosphate. Strong lighting is essential, the macroalgae can only work with what you give it.

Adequate live rock will allow for a significant amount of denitrifying bacteria which will remove nitrate. As long as you follow the normal guides for live rock (one pound per gallon minimum, two to three pounds per gallon is ideal) this should provide significant denitrification.

Water changes are also an important factor in nutrient control. Weekly 10% water changes should be enough to keep the nitrate under control (when used along with the other methods described here). If they aren’t enough, or something happens that throws the tank off balance such as a fish dying or overfeeding, then water changes every day for a week or more can be enough to get the tank back in balance.

Seachem Purigen in a media reactor can also be used to remove nitrate. Purigen removes nitrogenous waste at many different stages, so it not only removes nitrate but ammonia and nitrite before they even become nitrate. It also helps polish the water, which is a great added benefit. With any media reactor, the flow should be enough to keep the media tumbling. It shouldn’t just sit in the bottom of the reactor, but shouldn’t look like a tornado or be pressed against the top either.


Phosphate can also greatly encourage algae growth, as well as inhibit calcification. Phosphate should be almost undetectable (0.03-0.06ppm).

The same controls for nitrate should be used for phosphate (refugium, strong protein skimming, and water changes).

In addition, GFO (granular ferric oxide) used in a media reactor can also be a very effective tool against phosphate. As the name suggests, it is basically granules of rust. GFO removes phosphate and silicate.

Do not use aluminum based phosphate removers. It is obvious if they are aluminum based because they are white. In addition to medical risks associated with aluminum, these medias collect a bunch of phosphate and then dump it again if you don’t remove the media often enough (creating a sudden surge of the exact chemical you were trying to remove).


Alkalinity (KH), also called carbonate hardness, is the measure of carbonates and bicarbonates which control the pH and are used by stony corals and clams to build their skeleton. Alkalinity should be 8-12 dKH.

Baking soda, soda ash, buffers, and alkalinity additives are fairly straight forward and can be used as directed. When in doubt, go with less. It is always easier to add more later if needed than risk stressing livestock with big sudden changes in chemistry. The alkalinity should not be increased more than 1 dKH per day.

It is important to note that some alkalinity additives will change the pH, some won’t (or at least the impact will be minimal for the same dose). Standard baking soda will have a minimal impact on pH. Soda ash will raise the pH. If your pH stays around 8.4-8.6, stick with regular baking soda. If your pH stays on the low end around 8.0-8.1, then use the soda ash.

Add alkalinity and calcium at completely different times of the day to prevent them from simply precipitating with each other. Many people new to dosing will add them at the same time or only a few minutes apart. These same people can end up dosing very high amounts and still not get the change they should. This is because they are precipitating out of solution. I dose calcium in the morning and alkalinity in the evening. This gives the alkalinity a little bump before night time when the rate of photosynthesis drops due to the lights being off, which can allow CO2 to go up a little, which can cause the pH to dip a little. The extra alkalinity at this times helps to curb against this.


Calcium is used by stony corals and clams to build their skeletons. Calcium should be maintained at 380-500ppm. If you don’t have any stony corals or clams you don’t even need to test calcium. If you only have a couple small stony corals and are doing 10% weekly water changes with a high quality salt mix then you also probably won’t need to be adding any more, but testing is still a good habit to get into and will tell you when it is time to start dosing.

There are additives that can be used to increase calcium as needed. Many are sold as part of a two-part calcium and alkalinity combination since you effectively always have to add both if you need to add either. No matter what type of additive you use, it is vital that you are routinely testing your calcium. You will need to test a lot at first (daily), but over time you will figure out what your tank usually needs. At that point you can test less often (such as weekly) to make sure your dosing is correct.


Magnesium is also used by stony corals and clams to build their skeleton. Magnesium should be kept at approximately three times the concentration of calcium, 1350-1500ppm. Most people are concerned about alkalinity and calcium, but magnesium actually helps keep the other two from precipitating with each other directly in the water, so proper magnesium levels (at least 1300ppm) can greatly help keep the other levels stable.

There are additives that can be used to increase magnesium as needed. As with calcium and alkalinity, it is best to dose less rather than more until you get things figured out. Always test to make sure your dosing is correct. Magnesium usually doesn’t need to be dosed as often as calcium and alkalinity, so don’t be surprised if a dose once or twice weekly is adequate even if your other dosing is done daily.

Other Chemicals

There are other chemicals you may be told you need or see for sale in stores including iodine, strontium, etc. If you are using a high quality salt mix to do weekly water changes of at least 10% you probably will never need to even think about any of these.

Never dose anything that you don’t test for and know you need. Overdosing can be even more harmful than just letting the tank run without enough, especially with stuff like these trace elements. For example, you may hear that shrimp need iodine and if they don’t have enough they can’t molt properly. Although this is true, you probably have the right concentration already if you are using almost any half decent salt mix out there. If you dose without knowing what the iodine concentration is, you could easily bring it up too high which can actually cause shrimp to molt before they are ready. So unless you test iodine and see it is definitely too low, dose only as needed, and test again to make sure it is in the right range, don’t even think of buying a bottle of iodine.

In addition to all of this, most tanks aren’t so well balanced that things such as iodine and strontium will be your limiting factors. There is always something holding back your tank. Whether it is too much nitrate and phosphate, not enough light, not enough calcium, etc. there is always something holding it back. If you actually get to the point where all of these things are ideal and it really is something such as strontium that is keeping your SPS corals from being even more colorful, at that point buy a test kit and see. Until then, focus on the bigger, more likely problems first. Then, once you get that corrected, you can move on to the next thing. Keep things simple at first and slowly step up to more in-depth issues.


The good and bad of this hobby is that you can get very in-depth on some of these topics, and chemistry is definitely one of them. If you want to know exactly what strontium does and how it interacts with certain other chemicals, then there are articles out there that will explain it all. If you don’t want to get in to anything like that then don’t worry, you can still have a very successful reef without even knowing how to spell strontium.