What is DI water? How to get it and is it worth the investment?

In 2014, I began implementing RO-water (reverse osmosis). I didn’t have an RO Kit, but there was a window-washing facility nearby, and they sold RO water.

Getting all of the water I needed to my apartment was reasonably inexpensive, but it was quite a chore. So I looked at RO-kits, but they were a pain to set up because you need a place to install the kit permanently and have space for a big tank. It also takes a long time to generate the required amount, and it generates a lot of wastewater. 

The salesman at the window washing shop advised me to buy a water deionizer to produce DI (deionised) water . Simply connect a garden hose to it, and you’ll get pure water without producing any wastewater straight away.

The kit is a bit more expensive than an RO-kit, and also the cost of changing the resin bed is a bit more costly than exchanging the filters of an RO-kit as the resin bed has to be exchanged more often.

Benefits of DI water

  • The DI water has little to no solids compared to tap water. So you can match the exact water parameters you need for your setup.
  • Fewer algae. You can control the amount of NH4 and NO3 in your tank easier.
  • No wastewater during production compared to RO water.
  • Immediately ready, you don’t have to wait a couple of hours before you have enough purified water.
  • DI water has a low PH. It is easy to match the PH of your tank.
  • Less stress during water changes for animals and plants because you only add water with the same parameters as the aquarium water. This is very important if you use CO2. For example, the PH in my tank is 6,8, but the PH of my local tap water is 8,8. That is a drastic difference.

Differences between RO-water and DI-water

RO grade water, also known as Type III water, is the reversal of the natural osmosis process. Osmosis is the movement of water molecules from low ion concentration to high ion concentration through a semipermeable membrane. Our cells, for example, use the osmotic process to maintain the osmotic balance of the intercellular environment (s). Increasing the pressure on one side of the system is required to produce RO water. Feedwater is driven through semipermeable membranes by applying more pressure to one side, in this case, the untreated or dirty waterside. The RO process can typically remove 90-99% of contaminants.

Although not perfect, RO purification is a cost-effective technology because RO membranes can last for years if properly maintained.

DI grade water, also known as Type II water, is purified water that has had nearly all of its mineral ions removed, including cations such as sodium, calcium, iron, and copper, as well as anions such as chloride and sulfate. The DI method makes use of ion-exchange resins that exchange hydrogen (H+) and hydroxyl (OH-) ions for dissolved minerals, then recombine to produce water (H2O). Positively and negatively charged impurities in the water eventually displace all of the active H+ and OH- molecules on the DI resin, necessitating filter replacement. Another advantage of the DI purification method is that it can supply DI or Type II on-demand. Because the ion-exchange resin is not a physical filter with a pore size, bacteria and dissolved organic material will not be removed. Knowing your production water needs is critical when choosing a water purification unit.

Of course, using both is always is getting you the best result. And there are many options if you want to invest in a RODI kit. But if you don’t want to produce a lot of wastewater or have a kit permanently installed, and you need your water instantly, then a DI-kit is your best option.


Deionization tends to strip away both essential and harmful minerals, so you’ll need to remineralize your water to make it suitable for your fish. Of course, you can remineralize the DI water by adding Seachem Equilibrium or any other remineralizer on the market. Or you can just add tap water to remineralize your water.

You can measure the total dissolved solids you want to add to your water with the TDS meter. Because DI water has a lower PH (can be as low as 5,5), you need a PH test kit to determine how much you need to add to meet the parameters of your tank. Luckily you only need to do this once. So future water changes are less of a hassle. 

Water parameters of tap water rarely change. So no need for re-testing every time. You can get a test report from your local water supplier if needed. Usually, you can find them online.

I used both methods, but nowadays, I stick to just adding tap water to match the water parameters of my tank.

Cost of using DI-water

DI-water is a bit more expensive than the common RO-water used by aquarists. The kit costs, in most cases, much more than the average RO kit. Depending on your local tap water hardness, the resin bed has to be exchanged more often than the filters of a RO kit and is more costly.

I’m using the Unger nLite HydroPower DI Filter 1200L. The new model (UNGER DIUH1 HydroPower Ultra Filter) has an improved water flow through the resin bed. Resulting in a longer lifespan for the resin.


The unit, including one resin bag, costs about € 350,00 depending on where you live.

The unit is built very well and will last a lifetime. The only things you need to exchange are resin bags and batteries for the TDS meter.

Resin bags

How many resin bags you need depends on how hard your water is.

The formula to calculate that is 1L of resin can purify 1500L of water at 0ºdH. An Unger resin bag has 6L and can purify 6 * 1500L = 9000L of water at 0ºdH. 

To calculate how much water you can purify with 1 bag of resin, just divide 9000L by your local water hardness. Where I live, the ºdH is 7,7. So with one bag, I can purify 9000 / 7,7 = 1168,83L of water.
My water changes are 100L weekly, and I’m using 60% DI water and 40% tap water. 

I can do 20 water changes with one resin bag. So I need 2,5 resin bags a year. Where I live, a set of 4 bags of resin costs € 130,00. That’s € 32,50 per bag, € 32,50 * 2,5 = € 81,25 per year or € 1,56 per water change.

Conclusion Is it worth investing in an DI kit?

In my opinion, yes, it is 100% worth investing in a DI kit. I have been using it for years, and I don’t want to go back.

It made my life so much easier. The tank rarely has algae issues. After a water change, the animals and plants do not have to acclimatize to different water parameters resulting in less stress. I do not have to install an RO kit in my bathroom, and I do not generate unnecessary wastewater.