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This article is part two, read part one by clicking here.

Water Chemistry

Saltwater aquariums have very specific water parameters that need regular monitoring to ensure success. Below we outline key parameters to test:


Salt is essential in marine aquariums. Salt is measured either in parts per thousand (ppt, which is also equivalent to g/L) or specific gravity (SG). Measuring salinity is easily done using either a hydrometer (relatively inexpensive, with lower accuracy) or a refractometer (more expensive, but greater accuracy), both which should be available at your local aquatics retailer. The amount of salt needed varies slightly depending on the type of set-up you are going for, for a reef tank, the salt level should be 35ppt, or an SG of 1.025-1.026. This is close to natural sea water, and the higher SG allows higher levels of calcium and magnesium to be dissolved in the water, essential for healthy coral growth. Marine tanks containing only fish and live rock (FOWLR) can be kept at a lower salinity level of 1.020-1.025 (SG), some hobbyists claim the lower salinity helps the fishes’ osmotic regulation (less pressure on removing salt from the body) and marine parasites cannot thrive in lower salinity levels. Maintaining a lower salinity level would also require less salt for water changes.

Salinity levels change depending on water temperature. When mixing water ready for a water change, the RO water must first be brought to temperature before adding the salt. Salt remains in the aquarium when water evaporates, so if only topping up the tank be sure to check the salinity level first as it may only require a top up with pure RO water to maintain salinity. If salt water is used to top up the tank, the salinity will increase over time and reach dangerous levels.

pH & KH

Marine aquariums require a constant pH level to prevent any dangerous fluctuations. Over time, natural biological processes reduce KH and pH levels to lethal levels which can kill fish and invertebrates. The best way to maintain pH levels is to make sure the carbonate hardness (KH, measured as degrees carbonate hardness - dKH) level is high. In marine aquariums, the target pH is around 8.3. To maintain a pH of 8.3, it’s important to keep KH at approximately 10 dKH. Some hobbyists may suggest it is only important to test for KH, on the premise that a correct KH will create a correct pH.  Unfortunately, the carbonate hardness system is a little more complex. It is therefore recommended to check the pH at the same time as the KH.  If the pH or KH are incorrect, a good quality KH buffer, such as NT Labs Marine Buffer Powder, which contains a premium blend of carbonates and bicarbonates at the correct ratio, should be used to restore these parameters.

Ammonia, Nitrite and Nitrate

These three compounds make up the most important stage of the nitrogen cycle, nitrification, and is the primary reason for installing filters. Ammonia is the waste product produced by fish and other aquarium inhabitants and is very toxic, even in low concentrations. Filters aim to remove ammonia through biological processes and protein skimmers removing organics. Ammonia present in the water would suggest an insufficient amount of filtration, overfeeding, overstocking, an issue resulting from overmedicating, or a crash from broken equipment. Ammonia should always be kept at 0 mg/l.

Nitrite is processed in the second stage of nitrification and, whilst practically non-toxic in marine water compared to freshwater, should still be maintained at 0 mg/l in the aquarium at all times. Nitrate is the final product of nitrification and is also the least toxic. This should not mean it should reach high levels as corals and invertebrates can be adversely affected if levels rise above 10 mg/l. High nitrate can also fuel nuisance algae growth.

All three of these compounds should be regularly tested using the NT Marine Lab test kits to ensure healthy water quality.

Calcium and Magnesium

Calcium and magnesium are essential elements in the reef aquarium. Corals (both soft and stony), coralline algae and other invertebrates use calcium to grow. It is imperative to maintain good levels of calcium in the water but be careful not to overdose as too high levels of calcium will reduce KH levels, potentially causing a pH crash. Optimum calcium concentration in the water should be between 380 mg/l to 450 mg/l.

Magnesium is another essential element required in reef tanks. Although not directly used as a building block compound in their skeletons like calcium, invertebrates need magnesium in order to efficiently uptake calcium from the water. Correct levels of magnesium also stop calcium and carbonates from precipitating in the water. For ideal magnesium levels, it is measured as a ratio to the calcium levels found in the water. The ideal ratio is 3:1 magnesium to calcium. Therefore, if the calcium levels are maintained at 450 mg/l, the magnesium level should be 1350 mg/l.

Good quality salt mixes will provide optimum levels of calcium and magnesium at the correct ratio. When testing for calcium and magnesium using the Marine Lab Calcium & Magnesium Test Kits, if the values are found to be too low, a decent water change will re-establish the correct levels. In highly stocked reef tanks where calcium levels will quickly deplete, dosing the aquarium with a calcium supplement will reduce the need for extreme water changes. Some calcium supplements in the market will also raise magnesium levels to maintain the correct ratio of 3:1.


Phosphate in the marine aquarium can become problematic if left to reach excessive levels. High phosphate levels will encourage nuisance algae to grow, and in reef aquariums can inhibit healthy coral growth. Aim for as little phosphate in the water as possible, and in reef aquariums this should not exceed 0.03 mg/l. To keep phosphate levels low, use NT Labs Phosphate Remover in the filter or periodically dose Anti-Phos when levels are tested high.

Regular Maintenance Routine

The marine aquarium needs regular maintenance to stay healthy. Maintenance schedules can be divided up into daily, weekly and monthly.


  • Feeding - recommended to be done two to three times a day to help recreate a natural feeding regime for the fish. In the wild, fish continually search for food. By being offered food at different times this will aid their natural digestion times. Be careful not to overfeed, only feed as much as the fish consume in a few minutes. Overfeeding will lead to excess waste and poor water quality.
  • Health check of the fish - are they all behaving normally? Signs to look out for are: loss of appetite, ragged or clamped fins, staying in one location, cloudy eyes, parasites or red blotches. If something is not looking right and all water quality parametes are correct, please check out our diagnosis tool for help with which medication is most suitable.
  • Visual check of the equipment - are the pumps running as normal? Is there suitable movement at the water surface for good oxygenation? Is the temperature of the water within the suitable range?


  • Evaporation top up - ensure the water level in the sump has not gone down too much to cause any equipment to run dry. Only top up using pure RO water to not increase the salinity of the water. Fitting an auto-top up unit will ensure the fluctuation of salinity is not too much, periodically checking to make sure there is always an adequate supply of RO water.
  • Filter maintenance - if a mechanical filter sock is used in the sump, this should be checked and washed weekly. If the sock is soiled, water is not being adequately filtered and is simply bypassing into the sump. This will lead to a build up of waste in the bottom of the sump which is harder to remove.
  • Cleaning the aquarium glass using a myriad of glass cleaners. There are algae pads, pads on sticks, magnetic algae cleaners and even ones with metal blades.
  • In depth equipment check - emptying the skimmer cup, ensuring any salt creep is wiped from light units, weir combs are removed and cleaned.
  • Water testing - this is usually recommended to do before any cleaning to give a better understanding of the quality of the water. Testing immediately after a water change can give false readings and will not give a true representation of the aquariums water chemistry.


  • Performing a partial water change in the aquarium. Hobbyists have individual routines, but a good recommended amount to change is approximately 25% monthly. Using a gravel cleaner syphon to drain the water will also help lift any uneaten food and other detritus from decomposing in the substrate and creating excess waste. Replacement RO water should always be first brought up to temperature before adding the high quality salt mix to ensure the specific gravity of the mixture is correct.
  • Whilst performing filter maintenance it is recommended to check the moving parts of the filter, namely the impeller in the return pump. By removing and cleaning the impeller, it can improve flow rate, prevent clogging and extend the life of the motor.

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