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A Guide To Saltwater Aquarium Water Chemistry & Parameters

by Brian Dunleavy
A Guide To Saltwater Aquarium Water Chemistry & Parameters

Whether you are a beginner or expert fish keeper, it is important to understand basic ideal water conditions for your saltwater aquarium.

Water chemistry is one of the most important factors when keeping an aquarium, as it directly affects the health of your fish, corals, and aquatic plants. 

In this guide, we will cover the main water parameters inside your aquarium and what to do if the levels are off balance. 


Ideal Level: 7.8 - 8.6

The potential of hydrogen (pH) determines how acidic or basic (alkaline) your aquarium water is. 

Sudden fluctuations in pH levels can be fatal to fish because water enters their cells through osmosis, changing the pH of their blood. Maintaining the pH levels inside your aquarium will reduce stress on your fish and other aquatic animals and help them resist deadly diseases.

Problems With High pH

pH is one of those parameters that we often battle with to keep high enough in saltwater reef aquariums, however, if the levels are too high they soon become toxic to marine life. 

Lowering pH:

  • Add soda water
  • Add distilled white vinegar
  • Increase CO2 using a protein skimmer or other aeration method

Problems With Low pH

Low pH levels increase calcification and disrupt biochemical reactions. 

Increasing pH:

  • Use an algae scrubber in your filtration system
  • Use a CO2 scrubber

Alkalinity & KH

Ideal Level: 8 - 10 dKH

Alkalinity measures the amount of bicarbonate inside the aquarium water. This parameter is vital if you have SPS and LPS corals that build a calcium carbonate skeleton.Alkalinity also measures how well your aquarium water can neutralize acids and maintain the pH level.

Carbonate hardness (KH) is the number of carbonates and bicarbonates that bind to acids to neutralize them. This makes KH essential in preventing acidic water conditions. 

NOTE: KH and alkalinity are often used interchangeably

Problems With High Alkalinity & KH

High alkalinity and KH levels are linked to tip burning and rapid tissue necrosis in corals. For fish, high alkalinity and KH can strip their protective mucus layer, leaving them vulnerable to fish diseases. 

Lowering Alkalinity & KH:

  • Dilute the water via water changes
  • Often waiting it out is the best solution as minerals constantly become depleted in saltwater aquariums over time

Problems With Low Alkalinity & KH

As alkalinity and KH directly impact pH levels, a sudden drop can lead to drastic pH changes. 

Increasing Alkalinity & KH:


Ideal Level: 34 - 36 ppt (35 ppt is ideal!)

In saltwater aquariums, salinity is the general indicator for all dissolved compounds inside your aquarium. Most fish and corals can tolerate minor changes in salinity, however, significant changes can cause issues with osmoregulation and fish body metabolism. 

Problems With High Salinity

High salinity can be toxic to marine organisms. 

Lowering Salinity:

  • Dilute the water via water changes

Problems With Low Salinity

Low salinity levels often result in kidney failure in fish. Saying that, lowering salinity levels inside aquariums are often the “go-to” treatment for treating parasitic infections. 

Increasing Salinity:

  • Gradual water changes with saline water


Ideal Level: 0 ppm

Ammonia is toxic to aquatic life. A sudden spike occurs when uneaten feed or other organic matter decomposes inside your aquarium. 

Problems With High Ammonia

Ammonia levels greater than 0.25 ppm make it difficult for fish to breathe, eventually suffocating them, along with killing corals and invertebrates.

Lowering Ammonia:

  • Dilute the water via water changes
  • Add Seachem Prime 
  • Add heterotrophic bacteria

Problems With Low Ammonia

Low ammonia is not a bad thing unless you are in the middle of cycling your tank. In the very beginning, ammonia is important to fuel beneficial bacteria. 

Increasing Ammonia:

  • Add a source of organic carbon matter and let it decompose 


Ideal Level: 0 ppm

Nitrite is a toxic compound, hence the ideal level being zero!

It is toxic because bacteria convert nitrite to nitrate, another toxic nitrogenous compound. An increase of only 1 ppm can be enough to kill your fish within hours. 

Problems With High Nitrite

High nitrite levels are normal when you start a new tank, this is why you must cycle it properly before adding any tank inhabitants. Even the tiniest increase in nitrite levels affects the red blood cells in fish, and therefore their ability to carry oxygen is reduced, suffocating them to death. 

Lowering Nitrite:

  • Dilute the water via water changes
  • Add Seachem Prime 
  • Add heterotrophic bacteria
  • Add aquarium salt

Problems With Low Nitrite

There are no issues with low nitrite, as it should be zero. 


Ideal Level: <25 ppm (note, that nitrate levels depend on what you have inside your aquarium, so always check your tank inhabitants’ optimal levels)

As mentioned, nitrate is toxic, however, in small amounts, it is beneficial for aquatic plant growth. 

Problems With High Nitrate

Levels greater than 30 ppm are usually lethal to aquatic organisms. High nitrate levels also encourage excessive algae growth. 

Lowering Nitrate:

  • Dilute the water via water changes
  • Use an algae scrubber
  • Add heterotrophic bacteria
  • Dose with liquid carbon or distilled white vinegar

Problems With Low Nitrate

Low nitrate decreases algae growth, and you want some algae growth to keep your corals healthy!

Increasing Nitrate:

  • Gradually increase feeding
  • Add potassium nitrate
  • Add amino acids


Ideal Level: 0.05 - 0.15 ppm

Phosphate is the nutrient needed for photosynthesis inside your coral. 

Problems With High Phosphate

Too much phosphate increases unwanted algae growth. It can also discolor your corals, as the zooxanthellae that undergo photosynthesis are over-supplied. Increased phosphate levels also inhibit coral calcification rates. 

Lowering Phosphate:

  • Dilute the water via water changes
  • Add heterotrophic bacteria 
  • Use an algae scrubber
  • Use filter media (granular ferric oxide (GFO) is best)

Problems With Low Phosphate

Low phosphate is not too much of an issue. Low phosphate levels typically mean a lack of nutrients in the water. 

Increasing Phosphate:

  • Increase your bioload by increasing feeding without harming your fish
  • Dose with amino acids


Ideal Level: 400 - 450 ppm 

Calcium is a fundamental mineral for all saltwater aquariums.

Fish require calcium for neurological functioning, and stony corals, as mentioned, need it to build their coral skeletons. 

Problems With High Calcium

Too much calcium in your aquarium can clog your heaters and pumps.

Lowering Calcium:

  • Dilute the water via water changes
  • Wait it out

Problems With Low Calcium

Low calcium levels negatively affect the growth and overall health of fish and corals. 

Increasing Calcium:


Ideal Level: 1300 - 1400 ppm

Magnesium drives biological function and calcification. 

Problems With High Magnesium

Typically, high magnesium levels are well-tolerated in saltwater aquariums, however, marine invertebrates will start to die when it is too high. 

Lowering Magnesium:

  • Dilute the water via water changes
  • Wait it out

Problems With Low Magnesium

Low magnesium levels on the other hand are an issue! If the magnesium level is too low, it can negatively affect your fish and coral’s growth and health.

Increasing Magnesium:

  • Add magnesium chloride


Ideal Level: 76 - 80 °F (24 - 27 °C)

Now, I know what you are thinking. Temperature is not a chemical property of water. While you are completely right, water temperature is considered an extremely important water-quality parameter in aquariums. 

Problems With High Temperature

High temperatures cause coral bleaching to corals and heat stress, which is fatal to fish. 

Lowering Temperature:

  • Place a fan near the aquarium
  • Float ice in plastic bags on the surface
  • Use an aquarium chiller

Problems With Low Temperature

When the temperature drops, all metabolic processes slow down. 

Increasing Temperature:

  • Cover the aquarium in blankets/towels
  • Close any windows
  • Use an aquarium heater

Ways To Test Water Chemistry 

You cannot determine if your fish’s home is healthy by looking at it, so you need to test it. 

Ammonia, pH, nitrate, and nitrite water test kits are the best to maintain good water chemistry, but buying a good all-round tester kit is recommended to cover all bases!

We recommend the following testing kits:

You can also send off your sample in the mail, but this will give you delayed results, which could be an issue if you are experiencing a spike in your water chemistry that needs quick attention. 

Usually, we only recommend sending water samples by mail if you need specialized tests, for example, testing for heavy metals in your aquarium water. 


Understanding the water chemistry inside your aquarium will keep your marine life happy and healthy. Remember to maintain optimum levels by frequently testing the water parameters, and making adjustments where needed. 

If you need help with your water quality or reefing supplies, contact the Reefco team, or visit us in-store. We are always happy to assist fellow hobbyists!

by Brian Dunleavy