The first step in setting up a saltwater aquarium

The ocean is a highly stable chemical environment. The average salinity of natural seawater is about 35 ppt (approximately equal to 1.026 specific gravity at 25°C). Tropical saltwater fish, corals, and invertebrates have long lived in this stable environment.

 

Unlike freshwater fish, marine organisms must constantly regulate their internal salt balance (osmotic regulation). Marine fish actively excrete salt through specialized cells in their gills, while corals and invertebrates rely on a stable ionic environment to maintain cell function and bone growth. Rapid changes in salinity can cause: osmotic imbalance, increased metabolic burden, decreased immunity, and in severe cases, death.

In a home aquarium, the water body is a closed system. Evaporation continuously carries away pure water but not salt, causing the salinity to rise slowly. Regular salinity checks and water replenishment are necessary.

MUST ensure: 1. Salinity is close to that of natural seawater 2. Temperature is stable

Photo from Unsplash

Range

 

Common units for expressing salinity:

PSU (Practical Unit of Salinity)

ppt (parts per thousand)

SG (Specific Gravity)

Natural seawater has a viscosity of approximately: (at 25°C/ 77-78F)

35 PSU

35 ppt

1.026 SG

Recommended dosage ranges:

Pure fish systems: 30–33 ppt (1.022–1.024 SG) to reduce fish diseases

Mixed-species systems: 33–35 ppt (1.024–1.026 SG)

SPS coral systems: 34–35 ppt (1.025–1.026 SG)

Step 1: Prepare saltwater using DI/RO water

Tap water contains dissolved minerals, heavy metals, silicates, and other impurities, which may affect water quality and promote algal blooms, impacting the health of organisms. Therefore, RO/DI purified water must be used.

Correct Salt Preparation Steps

1. Add RO/DI water to a clean container.

2. Activate the circulating water pump.

3. Slowly add sea salt to the water (do not add water to dry salt).

4. Stir continuously for 2 hours or more.

5. Adjust the water temperature to approximately 25°C.

6. Measure the salinity and fine-tune to the target range.

(Note: Do not measure the salinity immediately after adding the salt, as incomplete dissolution will result in inaccurate readings.)

 

Step 2: Choose a suitable salinity meter

1. Optical refractometer/salinity meter (Beginner)

Principle: The concentration of dissolved salt changes the refractive index of light.

Advantages: Low price, Reliable, No batteries required

Disadvantages: Requires manual reading, Must be calibrated periodically

2. Electronic Salinity Meter

Principle: The conductivity of salt water changes with the concentration of ions.

Advantages: Digital readings, high accuracy (some can display temperature).

Disadvantages: Higher price, requires battery replacement, and regular calibration.

Step 3: Correct calibration (!!!)

Many enthusiasts use pure water for calibration, which is incorrect. Pure water can only calibrate the zero, leading to inaccuracies in the seawater range. (Note: A new salinity meter also needs to be calibrated with a standard solution.)

Correct Calibration Method

1. Clean the prism or probe with DI water.

2. Purchase and use 35 ppt standard calibration solution.

3. Fill the prism with the standard solution or completely immerse the probe.

4. Adjust the reading to 35 ppt or 1.026 SG.

5. Rinse and dry.

Step 4: Replenishing Water (During aquarium maintenance)

Evaporation only removes water, not salt. As the water level drops:

Salt concentration increases

Osmotic pressure increases

Biological pressure increases

To prevent a slow rise in salinity:

1. Mark the maximum water level on the surface of the aquarium.

2. Only use RO/DI water for top-up (do not use saltwater).

3. An automatic top-up system (ATO) can also be installed.

4. Check the salinity weekly or daily.

 

Photo from Unsplash

Summary

Before adding any organisms:

1. Prepare the saltwater using RO/DI water.

2. salinity close to natural seawater (34–35 ppt).

3. Properly calibrate the salinity meter.

4. Monitor regularly to prevent drift.

5. Maintain a stable temperature.

Reference

Boxman, S. E., Nystrom, M., Ergas, S. J., Main, K. L., & Trotz, M. A. (2018). Evaluation of water treatment capacity, nutrient cycling, and biomass production in a marine aquaponic system. Aquacultural Engineering, 83, 37–49. https://doi.org/10.1016/j.aquaeng.2018.07.003

Coker, D. J., Wilson, S. K., & Pratchett, M. S. (2013). Importance of live coral habitat for reef fishes. Reviews in Fish Biology and Fisheries, 24(1), 89–126. https://doi.org/10.1007/s11160-013-9319-5

D’Angelo, C., & Wiedenmann, J. (2013, December 29). Impacts of nutrient enrichment on coral reefs: New perspectives and implications for coastal management and reef survival. Current Opinion in Environmental Sustainability, 7, 82–93. https://doi.org/10.1016/j.cosust.2013.11.029

Common Question

1. How often should I change the water?

Regular water changes help maintain water quality. Many aquarists change about 10–20% of the water every 1–2 weeks.

2. What should I test in my water?

Common parameters include salinity, temperature, ammonia, nitrite, and nitrate.

3. Do I need to use RO/DI water?

Tap water often contains minerals, chlorine, or other substances that can affect marine organisms. Many hobbyists use RO/DI water to prepare saltwater because it provides more stable water chemistry.

4. How often should I test the water quality?

For a new saltwater aquarium, water parameters should be tested every few days during the cycling stage. This helps monitor the development of beneficial bacteria and the levels of ammonia and nitrite.

Once the tank becomes stable, most hobbyists test water parameters once every 1–2 weeks. Regular testing helps ensure that temperature, salinity, and nutrient levels remain within safe ranges for marine organisms.

5.How often should I do an ICP test?Beginner tanks: not required at the beginning

Established tanks: every 2–3 months

Advanced reef tanks: 3–4 times per year

ICP tests help detect trace elements and possible contamination in aquarium water.