Brackish water is saltwater mixed with freshwater to a specific gravity between 1.005 and 1.020

Brackish water sits between freshwater and full saltwater. You create it by dissolving marine salt into freshwater until you reach the right salinity level. Most home aquarists measure this with a hydrometer or refractometer — tools that tell you how dense the water is, which correlates directly to salt content. The specific gravity range of 1.005 to 1.020 covers most brackish fish and plants, though some species prefer narrower ranges within that band.

The process takes about 30 minutes from start to finish and costs very little. You need marine salt (not table salt or rock salt), a hydrometer or refractometer, a container for mixing, and a way to measure water volume. Once you understand the math, you can adjust salinity up or down by adding salt or freshwater.

Key Takeaways

  • Marine salt mixed into freshwater creates brackish water; table salt and rock salt contain additives that harm fish and plants.
  • A hydrometer or refractometer measures specific gravity, which tells you whether your salinity is in the right range for your species.
  • Most brackish setups use a specific gravity between 1.005 and 1.020, but check your fish or plant species for their exact range.
  • Mix salt into a separate container of water first, let it dissolve fully, then add it to your tank slowly over several hours to avoid shocking the inhabitants.
  • Test your water after 24 hours to confirm salinity has stabilized, because salt continues to dissolve and distribute.

What type of salt to use and why table salt does not work

Marine salt is the only salt suitable for brackish tanks. It contains the trace minerals and elements that fish and invertebrates need — calcium, magnesium, potassium, and others. Marine salt comes in two forms: synthetic (made from mineral compounds) and natural (evaporated from seawater). Both work equally well for brackish water. Common brands include when ready Ocean, Red Sea, and Fritz.

Table salt and rock salt are not safe. Table salt contains iodine and anti-caking agents that cloud water and harm fish gills. Rock salt is even cruder and often contains contaminants. Aquarium salt (sometimes labeled as such) is also not the same as marine salt — aquarium salt is usually just sodium chloride with no trace minerals, and it is meant for short-term treatments, not permanent brackish setups.

Buy marine salt in a quantity that matches your tank size. A 20-gallon tank needs roughly 1 to 2 pounds of salt depending on your target salinity. Most marine salt bags list how much salt per gallon raises specific gravity by a set amount — usually 0.001 to 0.002 per teaspoon or tablespoon. Check the bag label for the exact ratio your brand uses.

Measuring salinity with a hydrometer or refractometer

A hydrometer is a weighted glass tube that floats in water. The higher it floats, the denser (saltier) the water is. You read the specific gravity where the water line crosses the scale on the tube. Hydrometers cost $5 to $15 and are the most common tool for home aquarists. They are accurate enough for brackish tanks, though they can drift slightly if bumped or stored in extreme temperatures.

A refractometer is a small optical device you hold up to light and look through. A few drops of water on the lens show you the salinity reading inside the eyepiece. Refractometers cost $20 to $50 and are more accurate and faster than hydrometers, but they require a small learning curve to read correctly. Both tools work; choose based on your budget and patience for setup.

Before you use either tool, calibrate it with pure freshwater. A hydrometer should read exactly 1.000 in freshwater. A refractometer should read 0 parts per thousand (ppt) or 1.000 specific gravity. If it does not, adjust the calibration screw or dial according to the instructions. Calibration takes 30 seconds and ensures your readings are accurate.

Step-by-step process for mixing brackish water

Start by deciding your target specific gravity. Most brackish fish thrive between 1.010 and 1.015. Check the care sheet for your specific species — some prefer lower (1.005 to 1.010) and some prefer higher (1.015 to 1.020). Write down your target number.

Fill a separate container (a bucket, large pitcher, or spare tank) with the amount of freshwater you plan to add to your aquarium. Use dechlorinated water if your tap water is chlorinated — let it sit overnight or use a dechlorinator product. Measure the volume in gallons or liters so you can calculate how much salt you need.

Consult your marine salt bag for the dosing ratio. Most brands tell you how much salt per gallon raises specific gravity by 0.001. Multiply that amount by the number of gallons you have and by how many points you need to raise (for example, from 1.000 to 1.015 is 15 points). Add the salt to your bucket and stir for 2 to 3 minutes until it dissolves completely. The water may look cloudy at first — this clears as the salt fully dissolves.

Test the salinity with your hydrometer or refractometer. If it is lower than your target, add more salt a pinch at a time and stir. If it is higher, add a small amount of freshwater. Once you hit your target, pour the mixed water into your tank slowly — over 30 minutes to an hour if you have fish or plants already in the tank. This gradual addition prevents osmotic shock.

Adjusting salinity after the initial setup

Test your water again 24 hours after mixing. Salt continues to dissolve and distribute throughout the tank, so the reading may shift slightly. If it is still within your target range, you are done. If it is off by more than 0.002, adjust it by adding salt or freshwater in small amounts and retesting after another 12 hours.

Over time, water evaporates from your tank, but salt does not. This means the remaining water becomes saltier. Check your salinity weekly and top off with freshwater (not saltwater) when the level drops. This keeps salinity stable. If you notice salinity creeping up despite regular top-offs, do a partial water change — remove 25 percent of the tank water and replace it with freshly mixed brackish water at your target salinity.

Some aquarists use an auto-top-off system — a float valve that adds freshwater automatically as the tank level drops. These are optional but useful if you travel or forget to check the tank regularly. They cost $30 to $100 depending on the model.

Common mistakes that throw off salinity

Using the wrong type of salt is the biggest mistake. Table salt and aquarium salt will not give you the mineral balance brackish fish need, and they can cause cloudiness or algae blooms. Always buy marine salt.

Not letting salt dissolve fully before testing is another common error. Salt takes time to fully dissolve, especially in cool water. Stir for at least 2 to 3 minutes and wait 10 minutes before testing. If you test too early, your reading will be lower than the actual salinity.

Adding saltwater instead of freshwater during top-offs raises salinity over time. Only add freshwater to replace evaporation. Add saltwater only during scheduled water changes.

Forgetting to calibrate your measuring tool leads to readings that are off by several points. Calibrate your hydrometer or refractometer every month or whenever you suspect it has drifted.

Frequently Asked Questions

Can I use sea salt from the beach instead of buying marine salt?

No. Beach salt contains bacteria, parasites, and pollutants that can harm or kill your fish. It also lacks the precise mineral balance that marine salt provides. Always buy marine salt from an aquarium store or online retailer.

What happens if I add too much salt at once?

Your fish and plants experience osmotic shock — their cells lose water and they become stressed or die. This is why you mix salt in a separate container first, test it, and add it to the tank slowly over 30 minutes to an hour. If you accidentally add too much, do a partial water change when ready with freshly mixed brackish water at the correct salinity.

How often do I need to test salinity?

Test weekly during the first month to make sure your setup is stable. After that, test every two to four weeks or whenever you notice fish acting stressed. Always test after a water change or top-off to confirm the salinity is correct.

Can I use a kitchen scale to measure salt instead of a hydrometer?

A scale tells you how much salt by weight, but it does not tell you the actual salinity in your water. Different brands of marine salt have slightly different densities, so the same weight can produce different salinity levels. Use a hydrometer or refractometer to measure the result, not the input.

What if my hydrometer or refractometer breaks?

Buy a replacement. Hydrometers cost $5 to $15 and refractometers cost $20 to $50. Do not guess at salinity without a measuring tool — your fish depend on it staying stable. Keep a backup on hand if you can afford it.