If you’re setting up your first aquarium, moving from fresh into saltwater, or thinking of keeping extra-sensitive fish, you may come across the term “RO/DI water” in your research. What is this stuff, and should you be using it? If so, how—and why?
RO/DI water is a fantastic tool in the aquarist’s arsenal, and it’s pretty much a must for saltwater. Below, let’s find out what RO/DI is, why it’s so commonly used, and how your fish could benefit from it.
What is RO/DI water, anyway?
You’ve probably heard of RO/DI water before, though maybe not in the context of fishkeeping. This ultra-effective method of water purification is also sometimes used in home systems, after all. RO/DI means Reverse Osmosis/Deionized, and it’s a dual technique that yields pretty much the purest water known to man. Purer than distilled!
How does an RO/DI system achieve this ultra-pure H2O? The chemistry behind it doesn’t really matter for us aquarists in practice, but in case you’re curious, here’s the short version. As you’ll see, it should technically be called CF/RO/DI, as it consists of three steps:
- First, the water passes through a carbon filter to remove chemicals like chloramine, pesticides, and the like.
- Then, Reverse Osmosis is achieved by pushing water through a membrane that’s so fine, mostly just H2O pass through it. The rest—salts, minerals—don’t fit and are left behind.
- Deionization takes the RO water from “very clean” to lab-grade by eliminating anything that snuck past the membrane. The water flows through special resins that pick off any charged particles (calcium, nitrate, etc.) still present, replacing them with compounds that make water.
If the purification system works well, it’ll remove something like 99.9% of minerals, salts, and chemicals. RO/DI water is so pure that it’s suitable for use in pharmaceutics, cosmetics, the chemical industry, and the production of sensitive electronics like circuit boards. The technique is also used to make seawater fresh and potable.
And yes, RO/DI water is also pure enough for our fish tanks. It’s important to know, though, that it can’t be used as-is. That would kill your fish. We’ll see why below.
Why remineralization matters
If your tap water leaves a lot to be desired, your home may have its own RO/DI system. It usually comes with a little switch so you can choose between normal water (for things like washing dishes), and purified water, (for drinking and cooking).
But did you know that the water that comes out of these units is not RO/DI? The reason is that we actually want some minerals, just not the bad ones. RO/DI water isn’t nice for drinking, after all: it tastes flat. Due to its lack of salts, it can even mess up your body’s electrolyte balance if you regularly drink a lot of it. So, immediately after the unit produces RO/DI water, it introduces salts and minerals in desirable quantities.
Anyway, what we’re trying to make clear is that RO/DI water isn’t useful to living beings in itself. Our planet’s organisms, including humans and aquarium fish, need a variety of salts and minerals in their water in order for their cells to function properly. That’s where the advantage of RO/DI comes in: it’s a completely neutral base.
With RO/DI water, you can give even the most finicky fish, inverts and corals exactly what they want. And you don’t even have to figure out the mineral and salt quantities yourself! Reef salt mixes contain them in carefully balanced proportions tailored to marine life, and there are also remineralization mixes available for freshwater.
Did you know? You can check if water is RO/DI with a TDS (Total Dissolved Solids) meter. It will show a number close to zero, rather than the normal 50+.
Does my aquarium need RO/DI water?
Saltwater
We can be short about this: yes it does. Unless you’re buying premade saltwater, you should use RO/DI. Our tap water simply doesn’t contain the right concentrations of dissolved solids for sensitive creatures like corals and marine animals to thrive, and it can even contain compounds that actively hurt them.
Freshwater
For freshwater, it depends. Most folks can get away with using their tap water, though a water conditioner is usually required to at least get rid of the chloramine that is used to help disinfect tap water in a lot of places. There are two situations in which you may turn to RO/DI:
- Your tap water is seriously bad: it has high nitrates, heavy metal content, extreme hardness levels, and the like.
- Your fish or inverts are seriously finicky: tap water often doesn’t match the needs of highly specialized creatures like Sulawesi shrimp, wild-caught discus fish, and similar.
How do I use RO/DI water?
Well, as we’ve described, using RO/DI water properly is actually very easy. The hard part is in obtaining it: lugging around jugs can become old pretty quickly, especially if your aquarium is on the larger side.
We usually recommend the following:
- If your home already has an RO/DI system, you’re golden. When you need water for your aquarium, just take out the remineralization cartridge to obtain pure RO/DI. This can then be remineralized with reef salt or a freshwater-appropriate mix. Just don’t forget to put the cartridge back in when you need water for human consumption!
- If you have a small aquarium (like a 10-gallon reef cube or similar), you can buy RO/DI water or slightly pricier pre-mixed saltwater at your local aquarium store. It doesn’t lose quality when kept sealed and out of direct sunlight, so you can have them fill up big jugs that’ll last you a long time. Cost is not a concern, as you usually pay less than $0.50 per gallon.
- If you have a large aquarium, you could consider installing your own RO/DI unit. Basic systems, which are slow but work fine, usually cost less than $100.
As we’ve seen, once you’ve got the RO/DI water, all you need to do is remineralize it. For that, you’ll have to buy a mineral and salt mix appropriate for your tank. A basic reef salt mix should do the trick for most marine aquariums, while for freshwater, various brands exist for different species (Salty Shrimp for shrimp, Pro-Discus mineral for… well, it’s in the name).
We can’t give you specific dosage instructions, because those depend entirely on your goals—the remineralization product you’re using will tell you what to do. What we can mention is that a powerhead or air stone can come in mighty handy if you’re working with large amounts of salt, unless you have big biceps and don’t mind stirring vigorously for a while!
Lastly, before adding the remineralized RO/DI water to your tank, you should test it. Let the water sit for 30-60 minutes to allow excess CO₂—which influences acidity—to dissipate, then check. Even high-quality remineralization mixes can yield different values depending on aeration, RO/DI purity, and how the mix behaves.
Conclusion
When used correctly, RO/DI water allows you to fine-tune water composition to the needs of your aquarium. In marine aquariums, this isn’t optional, whereas in freshwater tanks, it’s rarely needed.
If all this talk about water purity and remineralization sounds like a hassle but you’d still like to have your own (saltwater) aquarium, we can help. FantaSEA Aquariums can design, build and maintain your dream tank for you—including the use of RO/DI water. That way, all you have to do is enjoy it! Contact us with your ideas and we’ll make them reality.
