I’ve been keeping saltwater aquariums for five years, and in all that time I’ve never used RO/DI water for top offs or mixing up clean salt water. I just use tap water treated with a dechlorinator / dechloraminator, and I’ve never had any problems. Could it be that people are exaggerating the benefits of using RO/DI?” – Submitted by Lee H.
Though you may have gotten by using only a dechlorinator/dechloraminator to treat your water thus far, you may not always be so lucky. I’m a strong proponent of using RO/DI-treated (or otherwise purified) source water in any marine aquarium system for several reasons. Here are just a few:
Water treatment protocols are for people
Municipalities aren’t (nor should they be) worried about providing water that’s safe for sustaining sensitive marine organisms. Their only concern is ensuring that the water is safe for people to drink, bathe in, etc. So, you can’t assume that whatever chemicals/additives are present (beyond the chlorine/choramine you’re treating for) are safe for fish and corals. What’s more, municipalities often change their treatment protocols with little or no public notice.
Bad things can happen to otherwise good tap water
Whether as a result of human tinkering or natural influences, previously safe, potable tap water can become contaminated. The current situation in Flint, Michigan is an obvious case of the former. An example of the latter occurred much closer to home—right here in Toledo, Ohio.
Located adjacent to Lake Erie, we’ve long enjoyed high-quality tap water in abundance. But in recent years, massive summertime blooms of Microcystis algae in the western basin of the lake have threatened the safety of our water supply. In fact, two summers ago, the problem reached crisis proportions when dangerous levels of microcystin (the toxin produced by Microcystis) temporarily rendered our water unsuitable for drinking.
In any such circumstance—or even less dire ones—I’d rather not take a chance that the tap water will be safe enough for marine life after merely treating it with dechlorinator/dechloraminator.
Less guesswork when problems arise
Whenever you experience a problem with your aquarium, such as a major algae outbreak, you have to go into Sherlock Holmes mode to figure out the cause. If you’re using RO/DI-purified tap water for top offs and water changes, you can safely rule out that the algae bloom is being fueled by nitrate, phosphate, or other nutrients present in your source water (assuming the membrane and various cartridges haven’t exceeded their functional lifespan, of course). In other words, with RO/DI, you’ve got one less thing to worry about!