Water chemistry in the saltwater aquarium
Water chemistry, or water quality is quite possibly the single most important thing you need to know about in this hobby. We don’t really think of what comes out of our tap; to humans it’s just water, but to fish it means everything. Think of it like the air we breathe. In saltwater aquariums, water chemistry is more complex than it is in freshwater aquariums. You should know how to test for pH, water temperature, ammonia, nitrites, nitrates, phosphates, salinity and for reef tanks even alkalinity, magnesium, and calcium.
IMPORTANT!
If your tap water is provided by the city and comes from a water treatment facility, your water must be treated with a water conditioner to remove all amounts of chlorine, chloramines and heavy metals that are extremely dangerous to your aquarium inhabitants. Although some people use tap water in their saltwater aquariums, I HIGHLY recommend ONLY using RO water for mixing new saltwater and topping off evaporation.
Always rinse your hands thoroughly before you place your hands in your tank. Cleaning chemicals could kill every living thing in your aquarium. This also includes soap, lotion and aerosol products.
pH (potential of Hydrogen)
This is how acidic or basic (alkaline) your water is. In the ocean, the pH of saltwater varies from 7.5 at the deepest parts to 8.3 at the shallow reefs. In our home aquariums we strive to keep the pH stable at 8.3, or as close to it as we can. Remember, stability is more important than attempting to reach a perfect target value.
Specific Gravity (or salinity)
The specific gravity, or SG is the concentration of salt dissolved in a body of water. In the home saltwater aquarium, our target SG is normally 1.020-1.026, with 1.025 being just about darn near perfect for a reef aquarium. With a FOWLR, you can get away with a slightly lower SG. I have seen FOWLR and commercial tanks as low as 1.017, but his would depend on the requirements of your fish.
Now I know what you’re thinking, what the heck kinda measuring cup is this guy using? To measure SG you can use either a swing arm hydrometer or a refractometer. I would not recommend using a swing arm hydrometer for any aquarium because I have personally seen hydrometers inaccurate up to +/- 0.003. Although more costly, a refractometer will be spot on once it is calibrated making it a much more reliable choice when testing the specific gravity of your salt water. This is one of those pieces of equipment that you should not skimp on. Your fish and corals will thank you later.
As water evaporates from seawater, salt is left behind. So as water evaporates from our aquariums, the salinity slowly increases as the salt becomes more concentrated. Because of this, we must “top off”, or replace evaporated water with RO water. For more advanced systems, an ATO, or automatic top off system is used to ensure the water level and specific gravity remains constant.
This is the waters natural ability to buffer its pH. You should test regularly and strive to maintain an alkalinity level around 2.2 meq/l (milli equivalents per liter). If the alkalinity in your tank is low, consider performing a water change to correct the problem, or use buffers to increase your alkalinity levels. Some substrates may also help keep alkalinity levels stable. Aragonite sand or crushed coral is often added to the aquarium for this reason.
Calcium
In shallow reefs as well as home reef aquariums, calcium is the building block for corals and invertebrates. They need it to grow their skeletons and protective shells. A good level of calcium for a reef aquarium is 450 ppm. To maintain this level, frequent water changes will replace the calcium that was lost. Between water changes, aquariums with a high uptake in calcium may benefit from calcium additives, or by installing a calcium reactor.
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Magnesium
Magnesium is needed for Calcium assimilation and proper osmotic functions in fish and invertebrates. When magnesium levels drop, so do calcium levels. Magnesium levels should be maintained at 1400 ppm (or slightly less for FOWLR tanks) with frequent water changes or magnesium additives.
Oxygen
Although you may not associate oxygen with water, it is still extremely important for our fish. Fish use oxygen for respiration just like we do, and just like us they convert that oxygen into CO2. As long as you have surface movement and disruption using a powerhead or filter, oxygen will find its way into your aquarium water, just as CO2 will find its way out through surface gas exchange. Another perk that the saltwater hobby has is protein skimmers. I’ll get into this more in when I talk about filtration, but for now just know that protein skimmers add plenty of oxygen into the water column for our fish to breath. One thing to remember is that as water temperature increases, the dissolved oxygen level decreases. This is something to think about if your aquarium overheats easily. This is one more reason why fans and aquarium chillers may be necessary equipment depending on the temperature of your tank.
Temperature
The temperature of a large body of water such as the ocean is quite stable year round. However aquarists are constantly battling heat issues in such confined systems as our home aquarium. It is important we monitor and stabilize the temperature of our aquarium with the use of heaters and chillers. The target temperature for most saltwater aquariums should be around 78ºF. You should research to find out what each of your fish prefer before adding them to your aquarium. Some hardier fish can be acclimated to live in water a few degrees warmer or cooler than their natural habitat. Again, remember that stability is the key.
The Nitrogen Cycle
Failure to understand the nitrogen cycle is probably the leading cause of failure for all beginner aquarists. Adding too many fish into an aquarium too fast will result in “new tank syndrome”. If you have had an aquarium before where every fish you seem to add ends up dying, it is most likely because the tank was never properly cycled. Without knowledge of the nitrogen cycle, it is easy for a newcomer to get frustrated and give up in this hobby. You don’t have to be a chemist or biologist to understand the basics of the nitrogen cycle.
Toxic ammonia is introduced into to water through decomposing uneaten fish food, fish waste and through fish respiration. As ammonia levels rise, aerobic bacteria begins to colonize on our aquarium rocks, decor, substrate, and within our filter. This beneficial bacteria converts the ammonia into nitrites, which is slightly less toxic, and then another type of beneficial aerobic bacteria converts the nitrites into an even less toxic compound called nitrates. Nitrates are the end result in the nitrogen cycle and we must physically remove them through frequent water changes or through nitrate reactors and refugiums (for more on reactors and refugiums, see equipment). Aerobic bacteria thrive in high oxygenated water, we just have to sit back and wait for them to go to work. It takes roughly 6 weeks from the time you first introduce an ammonia source into your aquarium for the nitrogen cycle to be complete and ready to be slowly stocked with fish.
The easiest and most humane way to cycle a tank is called a fishless cycle. What this means is we provide an ammonia source (such as adding a raw shrimp and allowing it to decay) and waiting for the nitrogen cycle to complete. During this time you should not perform any water changes or add any fish and you should be testing for ammonia, nitrites and nitrates every few days to give you an idea of which stage the cycle is in. When ammonia and nitrites fall to and remain at zero, you may now do a water change and slowly start stocking your tank. And please don’t forget to remove that raw shrimp. Some people also do what is called a natural cycle using a hardy fish that can tolerate high levels of ammonia and nitrites. Although I have done this in the past, I would suggest doing a fishless cycle so you don’t put any animal through the stress and discomfort of a cycling tank. There are a few ways to speed up the nitrogen cycle if you happen to be as impatient as I am. There are some commercially available products that add the bacteria to your tank, but I have never had much luck with these products. The method I use is transferring sand, rock or filter media from a mature, already cycled aquarium into my new aquarium. This will introduce beneficial bacteria and allow them to colonize more quickly.
Water changes
In nature, toxins and waste are diluted and filtered out over millions of gallons of water. In our home aquarium however, it is our job to ensure our fish have the cleanest water possible. We do this through water changes. I would suggest changing 10-25% of your aquarium water every other week to once a month, depending on the combined bioload of your inhabitants, to keep your fish and coral happy and healthy. (Check your nitrate levels often; when you see the nitrate levels reach 25 ppm, it is time for a water change). Some inhabitants that are very sensitive to nitrates may require you keep the levels even lower than 25 ppm. Conducting a water change is really not that difficult, so don’t get lazy and slack on it.
Before a water change, you should have already mixed up a fresh batch of pre-mixed saltwater to the desired specific gravity a day in advance. Large trashcans work well for this. (Please ensure that you ONLY EVER use these containers for aquarium use).
First, unplug your filter pumps (and heater and any other equipment that may become exposed as the water level drops). Next, siphon out 10-25% of your tank water with an aquarium gravel vacuum into 5 gallon buckets. While siphoning, you need to be careful not to siphon up the sand, so just lightly move the vacuum above the sand to pick up any debris sitting on top. Once you have removed the desired amount of the water you can discard the old tank water and with a cup or pitcher, gently pour the new pre-mixed saltwater into your aquarium. Once it’s filled back up, plug your equipment back in and you are done with your water change.
Where can I find more information about aquariums?
Epic Aquarium is a site that teaches beginners how to set up an aquarium. Learn about types of aquariums, equipment, how to set up an aquarium, stocking fish, controlling algae and more!
As an aquarium enthusiast, my goal is to give back to the comunity and help others have an aquarium they can enjoy. I am the author and owner of Epic Aquarium.
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