You may have heard of reverse osmosis and deionized water filters for your aquarium. But do you really need them? First we’ll take a look at just what these gadgets are and discuss their value for your aquatic pets.

Reverse Osmosis

You may have heard the term osmosis as a kid when your teacher told you you weren’t going to learn by osmosis, you had to open and read the book! Reverse osmosis removes impurities in your water through the use of very fine, semipermeable membranes. Only the purest water can pass through.


Deionization removes ions in the water impurities such as dissolved salts, calcium ions, magnesium, etc, and then replaces these ions with hydrogen ions.

Do you need water purification?

Essentially, both reverse osmosis and deionization are water purifying techniques. If you’re relying on tap water to fill your tank, test first to see just how many impurities are in your water with a device called a TDS (Total Dissolved Solids) Meter. THis will tell you how many “things” are in your water. Chicago tap water is usually under 150 TDS.. Additionally, some level of minerals and essential elements in your water are needed, and you may end up adding these back in to reverse osmosis/deionized water. If you have a freshwater tank, it’s not typically likely you need either of these devices, but we’ve also seen tap water which is flat out toxic to freshwater fish too.

If you are keeping a saltwater fish tank, a reverse osmosis filter may be essential, depending on the water coming out of your tap. For a saltwater reef tank there is no doubt, anywhere in the country you should be using RO/DI water. Additionally, you could purchase reverse osmosis water, often found in pet stores.

The short answer to whether you need reverse osmosis and deionization filters is “it depends, but certainly never a bad idea or might be critical.” It all boils down to the requirements of your aquarium and your water sources. If you have any questions, get in touch with us for more info!

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