It is safe to say that most people use their tap water in their aquariums and probably couldn't tell you what's in it. The main parameters we look at in an aquarium are generally GH (general hardness), KH (alkalinity) and PH (potential of hydrogen). If you'd like to learn more about water chemistry, check out our Water Chemistry video.
What does an RO/DI system do?
Basically, a RO/DI unit is a filter and it strips the water of virtually everything but the H2O allowing us to then add minerals back in to meet our needs. This not only removes all the chemicals and organics in the water (more on this later), but also gives us full control over the GH, KH and PH.
Why would one want to strip their water with a RO/DI system?
Well, not everyone's tap water is created equal. We hate to see people online say, "I just use tap water", as if everyone's tap water was the same. One situation where you'd want to use a RO/DI system is if you had really hard water with a GH of 28, but wanted to keep soft water fish, like a discus. You'd need a RO/DI unit to stip all the minerals out of the water, and we can add back minerals to get the GH back up to something more reasonable, like a GH of 8. Another situation in which you'd want to use a RO/DI system is if you had unwanted organics in your tap water. Like if out of the tap you had a lot of ammonia, nitrates, nitrates, or phosphates. For your livestock health, and/or to help prevent algae, it's not wise to be dumping a bunch of ammonia or phosphates into your tank with each water change. So again, that is when you'd want to go with a RO/DI unit and make your own water. RO/DI units also more all the chemicals that are added in for public health reasons. Like Flouride. Your fish DEFINITELY don't need to be sucking on fluoride on a day-to-day basis. lol.
Is there anything you have to do to RO/DI water to make it safe for fish?
Once you strip the water of everything, you'll need to add essential minerals back to make it safe for livestock and plants. How much mineral content you put back is dictated by your plants or livestock. To raise the GH in RO/DI water, I suggest a product called Seachem Equilibrium. To raise your KH, I would just plain ol' baking soda, which is sodium bicarbonate. If you need your PH to raise with your KH, use a carbonate (not to be confused with bicarbonate) like soda ash, or sodium carbonate. With a little Equilibrium and baking soda, you have COMPLETE control over your water parameters. It's really that easy and can avoid TONS of issues that come with using municipal water supplies, or even well water. Both can be really unstable and uncontrollable.
What RO/DI unit do you recommend?
The RO/DI unit that we suggest is a RO Buddie by AquaticLife. I have had a 50gph unit for years now and never had an issue. At around $60 you can have complete control over your water parameters making sure your water parameters are rock solid. This connects up to a typical hose spigot or home faucet and can save you time and money from grabbing gallons of distilled water (which is the same thing) from your local department store.
To answer the question, do you need to RO/DI water? That answer is, only if your tap water is not suitable for aquarium keeping, OR, if you want full control over your water parameters when keeping something like sensitive fish or shrimp.
I hope this helps! :) Happy aquascaping to you all!