Making Reverse Osmosis Water Suitable for Aquariums

Why is Re-mineralisation Necessary?


While reverse osmosis and de-ionization filtration removes excessive levels of nitrate, phosphate, heavy metals and chlorine that aquarists do not want in their water source, it can also remove some of the essential elements. Therefore, reverse osmosis water must be buffered and “re-mineralized” before being used in any aquarium. Otherwise, the lack of minerals creates a poor environment for plant growth, fishes’ survival and development of your aquarium’s microbial communities.

This issue can be easily solved by using a re-mineralising buffer and these are available for both fresh and salt water use, enabling the fish keeper to produce the correct water parameters for every kind of fish. Many stores can supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added.

Minerals in your Water

Before you make any adjustments to your reverse osmosis water, it would be useful to understand the different components of water. A stable environment is key to ensuring the long-term health of our valuable marine pets. These are some of the key factors that affect the suitability of your aquarium:

  • Potential of Hydrogen (pH) and is the measure of how acidic or alkaline a solution is. At a molecular level, this refers to the concentration of hydrogen ions (H+) in a solution. Below pH 7 means that a solution is acidic while a pH level above 7 is alkaline.
  • GH refers to General Hardness and measures calcium and magnesium as well as certain other minerals. GH alone does not influence pH.
  • KH measures the amount of carbonates and bi-carbonates in the water. It is referred to as a buffering capacity as it prevents drastic changes in pH values. KH directly influences pH as the higher KH levels will reduce drastic pH changes.

Reverse Osmosis water has zero GH and KH as well as a pH from 6.2 to 7.5. Before re-mineralising your reverse osmosis water, you must conduct ample research into the water parameters required by your chosen fish.

You should also work towards closely imitating the water conditions of your fish’s natural habitat. You can refer to this link which elaborates on the water parameters for each kind of fish:

Conditioning your Water

It is crucial that you avoid any sudden changes to pH, GH and KH. Many species of fish and coral will be adversely affected by an increase or drop in pH by 0.3 and this can even prove to be fatal for some.

You should avoid chasing “perfect” values for your pH, GH and KH levels. It is very difficult, in practice, to tailor the exact details of your tank’s water parameters to the needs of your fish. This is because the tank requirements of fish may vary depending on their species and whether they have been wild-caught or commercially bred.

Therefore, you should instead focus on maintaining a stable environment and water parameters, without adding too many substances and agents to the water. This will greatly benefit the growth of your aquarium’s inhabitants. These guidelines would also help to reduce any unnecessary stress on your fish and corals.

Avoid Sodium-based Agents

Home water softeners that employ either sodium chloride or potassium chloride should never be added to aquarium water. These compounds strip essential calcium and mineral ions while increasing sodium to very high.

This can ruin the balance of minerals in your aquarium as sodium is only required in trace amounts for most fish. Elevated levels of sodium can severely affect osmoregulation in fish, especially in fish such as Loaches that normally prefer “softer” water. Some aquarists have even used baking soda, which is the buffer sodium bicarbonate. We would not recommend using baking soda because the purity is questionable and the dosing may be unreliable.

Re-mineralisation for Saltwater Aquariums


If you are serious about your reef tank, many aquarists advise upgrading to a reverse osmosis water purification system so that you can start with pure freshwater.  Once you have bought your sea salts, you can start mixing it with your reverse osmosis water.

Unfortunately, there is no standard amongst sea salt manufacturers as to how many pounds of sea salt to add to a given volume of water. This is because every brand is slightly different from each other.  You should clearly follow the instructions on the brand of sea salt you bought in terms of pounds of salt to add per gallon or 5 gallons of water to get the right salinity.

You should not mix sea salts in your aquarium as the salts do not dissolve quickly and it takes a lot of checking and time to figure out if the salinity is correct.  Unless your tank is completely empty then you can mix the sea salts in the tank. If not, you should mix your sea salts with the reverse osmosis water in a bucket.

Here are some guidelines to mixing sea salts into your reverse osmosis water:

  1. Fill your bucket with a known volume water.
  2. Add a measured amount of sea salt. You can add 3 to 4 times more sea salt to the water in the bucket than the recommend final amount.
  3. The more sea salt you add, the longer it will take to dissolve. However, you will not have to mix as many buckets of mixed salt.
  4. For most saltwater and reef aquaria, the temperature should be from 76 to 80 degrees Fahrenheit. Do not worry about getting the temperature exact!
  5. Also remember that there is no one absolute correct salinity value. It is important to just get into a range. The salinity range should be from 1.020 to 1.024 on your hydrometer.
  6. If the water is too salty, you need to remove tank water and add more freshwater.
  7. If the salinity is too low, remove 5 gallons of tank water and transfer it to a bucket. Then, you can add some sea salts into the bucket.  The amount of sea salt to add depends on your situation. You will have to estimate and re-measure as you go along.

Overall, when it comes to adding minerals to your reverse osmosis water, there are no universal measurements of agents to add as each brand is different. Instead, the process of re-mineralisation works on a trial-and-error basis. You should focus on keeping to a range of values as specified by the company’s brand as well as avoid adding too many buffering agents which could undermine the stability of your aquatic environment.

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