Anemone of a Lame Aquarium: TDS - Not a Satirical News Show

Science is the foundation of everything - that is why it should be no surprise that the same is true for keeping a reef aquarium. In an aquarium everything starts with water, and what is in it. This is what we will be examining today.

It is not as simple as going to your faucet, mixing in some salt and "EUREKA!" - tank-ready water. We'll reserve the "how to make high-quality salt water" discussion for a later day. For now, we'll take it one step at a time...

Purity - clean pure water is essential to begin the process. Tap water just doesn't cut it - even when you use a conditioner. It is full of dissolved minerals and organics that can cause harm to fish and corals. Total Dissolved Solids (TDS), as they are called, are found in all untreated water and they are measured in Parts Per Million (PPM) with a TDS meter, like the one shown below.

Luckily in NYC the TDS are relatively low - 45 ppm
Some places have water over 500 ppm

Since we want the absolute minimum amount of TDS in our water, we need to figure out a way to get rid of them. Sure, you can buy pure deionized water at the grocery store, but at ~ $ 0.75/gallon, the cost will add up when establishing a large volume ecosystem. Plus, why buy fancy water when you can buy fancy equipment to make your own! That is where a RO/DI (reverse osmosis/deionization) system comes into play.

According to http://www.reefcentral.com/index.php/rodi-faq this is how it works:

There are typically four stages in a RO/DI filter: sediment filter, carbon block, reverse osmosis membrane, and deionization resin. If there are more, something was duplicated. (In my case the deionization resin.)

The sediment filter removes particles from the water. Its purpose is to prevent clogging of the carbon block and RO membrane. Good sediment filters will remove particles down to one micron or smaller.

The carbon, typically a block of powdered activated carbon, filters out smaller particles (ideally down to 1/2 micron or smaller), adsorbs some dissolved compounds, and deactivates chlorine. The latter is the most important part: free chlorine in the water will destroy the RO membrane.

The RO membrane is a semi-permeable thin film. Water under pressure is forced through it. Molecules larger/heavier than water (which is very small/light) penetrate the membrane less easily and tend to be left behind.

The DI resin exchanges the remaining ions, removing them from the solution.

Because I need to supply lots, and lots, and lots, of pure water to my system -- a 90 gallon system may take 3 times that to set up + a 20% water change every other week-- I decided to buy a 5-stage unit that is rated 150 gpd (gallons per day) for $143. This system should perform well, considering NYC's water has such a low TDS, producing as much as 1,000 gallons before filter changes are necessary.

5-stage 150 gpd RO/DI from Water General

It only takes basic plumbing skills to get the unit up and running... or so I thought, right baby? You might want to locate your nearest plumbing/hardware store before beginning installation, just in case.