Fun topic! I’ve been DIYing reverse osmosis water for a while now, it’s definitely a rabbit hole but with some definite rewards. There’s a bit to unpack here… - In my experience, ion exchange is used to treat hard water (water with a lot of calcium) by swapping out the calcium with a different ion (for example, sodium or potassium). The water that my home system generated was safe to drink, but is definitely not the same as distilled water. - Distilled water is just H2O, no hardness, no alkalinity, so other dissolved substances. Think of it as a blank slate for creating whatever water chemistry you wish. If available, reverse osmosis water is largely the same. You may be able to purchase RO water from a local company that supplies water coolers or installs filtration systems. For example, where I live I can refill a 5 gallon jug for $1. -Deionization is different from ion exchange. I don’t know much about deionized water, just that we used it in chemistry labs and were told not to drink it. The stuff in a grocery store may be different, have fewer harmful impurities. You’d probably need to research each brand. - If distilled/RO isn’t available, you might look into whether your local stores sell “spring water” or “artesian water.” This will contain hardness/alkalinity/dissolved solids. So a bit more complicated for DIY, but some people have researched/posted recipes for mixing certain brands to active desired water chemistry. - On that note, you don’t need to start with chemically pure water to DIY brew water. There are some excellent calculators online that allow you to plug in a starting water chemistry, and calculate what you need to add. This of course assumes that your tap tastes good and doesn’t already have excess concentration of the parameters you care about. Good luck, it’s a fun road ahead!