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Most people keeping octos use RO/DI water. But the book Water Chemistry for the Marine Aquarium by John H. Tullock says that distilled water is just fine to use, but more expensive than RO/DI water.
When I read this, I began buying 1 gallon containers of distilled water for my topoff water, and it made my life a lot easier!! (no more pouring from 5-gallon containers for topoff) I use a fair amount of topoff because I have a fan blowing on the sump. It seems to have worked just fine.
Distilled water is so stripped of everything, I'm surprised that it's advisable for use in an octo tank. You would not use distilled water in a reef tank (yes, I know we're not talking reefs). My reef-based mind keeps getting jolted by you ceph people.
Is distilled ok for octos because you don't want any type of mineral in the water?
Colin--I see what you're saying. But, as you know, there are other things in sea water other than salt. Plus, the salt you buy for your tank has been purified, so-to-speak. It is not simply evaporated sea water. I use r/o water, which is not as stripped as distilled. Distilled is so pure, many nutritionists recommend spring water or standard filtered water instead. Either way, I don't think this is a major issue. Just curious.
With spring water, the minerals are not removed (possible biological contaminants are not either, though). Some of the aforementioned minerals are rare, difficult to replace, etc. Plus, we aren't advanced enough to know which are truly helpful, and which are not. In all, it's perfectly natural.
With distilled water, you are left with absolutely pure H20. That's it. No added minerals, etc. It is all lost. That's fine if you are ironing a shirt, but in a reef tank, you always want the mineral content to be strong, for the corals and other similar creatures. They absorb the beneficial elements that are difficult to add otherwise. The "mineral/vitamin" additives on the market are a joke, needless to say.
With R/O water, you maintain a strong mineral component (but lose a considerable amount), and eliminate much of the biological pollution in most city water. Buying spring water is obviously much more expensive than using R/O in-home. Personally, I'd prefer to use spring water exclusively.