Deionized Water

Deionized water is very pure and it is what we should be using for our saltwater aquariums, be it for evaporation top-off or to make up new saltwater for water changes.

Deionized Water | Deionization Filters

Deionization (DI) filters usually consist of three stages — an activated carbon filter and two stages of a deionizing resin.

Some aquarists incorporate a a sediment filter as well, although I personally find this to be unnecessary.

Water from a tap is made to flow slowly through the activated carbon stage before it reaches the deionizing resins. The activated carbon removes chlorine and chloramines, taking some of the load off of the deionizing resins, allowing them to last longer.

The flow of deionized water exiting the DI filter should only be slightly more than a trickle. Too high a flow rate and there will not be enough contact time between the raw tapwater and the resins, resulting in a TDS (Total Dissolved Solids) reading higher than zero, even with brand new resins.

DI filters remove impurities from tapwater by a process of ion exchange. Deionizing resins are ‘hungry’ for impurities like silicate, phosphate, ammonia, nitrates, minerals, chlorine and chloramine as well as heavy metals like copper. Basically all the stuff you don’t want in your reef tank!

Should I Use An RO/DI Water Purification System?

RO or Reverse Osmosis water is produced by forcing tapwater under pressure through a semi-permeable membrane. The RO membrane captures much of the impurities present in tapwater. An activated carbon filter after the RO membrane removes chlorine and chloramines that are not removed by the reverse osmosis process.

In an RO/DI system, the water is first purified by reverse osmosis before final purification by deionization.

Processing tapwater by reverse osmosis is however, a very wasteful process with only about 10 to 15% of the water making it through the RO membrane at high pressure. The rest of the water is discharged as effluent which can be collected and used for other household purposes.

If you are purifying water by reverse osmosis for a large 200 gallon aquarium, you will probably have more effluent water in storage bins than you can use!

Using a simple DI filter, on the other hand, produces no waste water. 100% of the raw tapwater put through a DI filter is processed into pure deionized water.

Another major downside to reverse osmosis systems is that expensive RO membranes will need to be replaced every few months. While DI resins will also need to be replaced when exhausted, they are nowhere near as expensive as RO membranes.

Reverse osmosis units available for the aquarium hobby produce 25 gallons per day up to 100 gallons per day, depending on the capacity of the unit.

Use an RO/DI system by all means, but be aware of the drawbacks compared to a simple, hassle-free DI filter system for producing deionized water.

Deionized Water | When Do I Replace My DI Resin Cartridge

This depends on the tapwater quality in your area. A TDS tapwater reading of 50 will mean that your deionization resins will last longer than if your TDS tapwater reading was at 100!

The raw tapwater TDS reading in my area is about 65. Once filtered, the deionized water leaving my 3-stage DI filter reads at zero..

When the water exiting the DI filter starts to read around 5 or 7, I usually find that the DI resin starts to get rapidly exhausted soon after that, with readings of 50 and climbing.

Which is why the TDS meter is your best friend when checking for DI resin exhaustion. Once DI resin is exhausted, it will start to release the impurities it has adsorbed.

Always check the condition of your resins by measuring the Total Dissolved Solids coming out of your DI filter every time you make new water! Remember it is no longer deionized water once your TDS reading is anything more than zero.

Although some DI resins change color to indicate when they are exhausted, nothing beats a TDS meter for actually seeing what is actually in the water exiting your deionization filter.

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