RODI Systems — Do You Need One?

Most experienced aquarists consider this THE most crucial piece of equipment for a saltwater aquarium — and yet, it almost always hidden in the background… no one ever sees it… and many people just starting out in the hobby have never even heard of it…

What is this magical — and often expensive — piece of equipment?  It’s a Reverse Osmosis Deionization system, or RODI system. Or, put simply, a water filtration system.

Essentially, water flows through one to three filters to remove chlorine and larger mineral and sediment particles, then through a reverse-osmosis membrane to purify it further, and finally through a deionization cartridge to remove the few, final impurities that were left.

I am by no means a biologist and have no intention of trying to boring you with any more details on how RODI systems work. If your curious you can find a ton of info on forums like

The big question is:  Do you need an RODI system for your saltwater aquarium?

It depends…

It goes without saying that reef aquariums are very sensitive and complex ecosystems — even very small traces of chemical (or mineral) impurities can quickly trigger very nasty side effects and cause a great deal of harm. When adding fresh top-off water to your tank, or mixing new saltwater to do your water changes, it is very important to use water that is close to pure H2O as you can get.

With that said, you have three choices:

  1. Use your tap water and take a BIG risk of poisoning your system (The chlorine/chloramine alone in tap water, and the heavy amount of minerals in most well water, can cause significant harm to the inhabitants of your reef tank)
  2. Make frequent trips to the local pet store to purchase jugs of RODI water. The larger your aquarium, the more jugs of RODI water you’ll have to regularly haul home
  3. Get an RODI system which, while initially expensive, can quickly pay for itself several times over in the form of time it saves you, livestock it keeps healthy, and gas and money saved on trips to the pet store

What kind of RODI system is right for you and where can you get one?

RODI systemRODI systems are based on the amount of purified water they’re capable of producing in one day. For example, under typical conditions a 100GPD system will produce 100 gallons of pure water every 24 hours.

The size of your aquarium will determine the what size RODI system you will need.

And your water source will determine what type of filters it will contain.

If you’re on a city water supply, you’ll need to ask your water company whether they use chlorine or chloramine. A RODI chlorine cartridge will not remove chlorine, so you need to choose an RODI system with the right type of cartridge.

It may also be worthwhile to ask them what the Total Dissolved Solids (TDS) of your tap water are. They do regular testing of their water as a matter of public safety and will share the most recent test result with you if you ask.

If you are on well water, you may want to consider a cistern or container to some well water in for a few days before running it through your RODI system. The higher levels of CO2 in well water can wear out the reverse-osmosis membrane much quicker than usual and it is the most expensive cartridge to replace on a RODI system. Allowing the well water to sit for few days before filtering it will allow some of the CO2 to naturally degas out into the air before you filter it.

Whichever RODI system you choose…

Make sure it comes with a TDS meter!

It’s important to measure each batch of finished water to ensure is has a TDS of zero (or at the very least less than 4, although it’s best to have 0 TDS). When your TDS starts to rise above that, you’ll know it’s time to replace one or more of the filters.  Any respectable RODI system will, at the very least come with a hand-held TDS meter so you can test your finished water. Many good RODI units come with one or more TDS meters built into them (so you’ll know which specific cartridge needs to be replaced).

I realize this all may seem a bit confusing at first, but once you look at a few it starts to become a bit more simple…

I suggest starting your search on websites the of Spectrapure, Buckeye Hydro, and Air, Water, and Ice — all of whom are makers and sellers of highly respected RODI systems. Then, check out online saltwater aquarium stores such as and and read reviews of some of the RODI systems they sell.