One question I see frequently is, "Do I need ro/di water"? Of course the short answer is "yes", but here's the scoop, because it might help if you understand the chemistry of sea water. I'm not going to discuss nutrients or heavy metals, because that's a given. There is much more... Natural Sea Water: Over millions of years the oceans evolved until they finally stabilized at what they've been for the last several hundred thousand years; their waters have a consistent blend of many, many dissolved minerals, elements, and compounds ranging from several PPT (parts per thousand) to less than 0.001 PPB (parts per billion). At the same time, the sea's delicate creatures have also evolved, developing a tolerance for the elements to which they are exposed, at the levels found in NSW. Artificial sea salt is designed to mimic NSW as closely as is humanly possible, as all of those elements have been added, but that end is impossible without completely purified water. Tap Water: NO tap water is "good enough". Regardless of the source, ALL tap water also contains dissolved minerals, elements, and compounds. The elements found, and their levels, may vary depending on one's location or the source, but if used "as is" to mix artificial sea water, it WILL disrupt the delicate balance of natural minerals, and cause unnecessary stress to their pets. Even for those who seem to be "getting away with" using straight tap water, over time it will affect their pet's health and lifespan. Additives and Tap Water Filters: Most additives only remove chlorine and chloramines. Some do neutralize the toxicity of heavy metals, but none of them do anything for the other minerals or nutrients. Tap water filters, which basically use carbon for chemical filtration, remove nutrients and lower heavy metal content, but again, have no affect on dissolved mineral compounds. To sum it up, save yourself and your pets from a migraine and shorter lifespan: buy an ro/di filter for your tap. Hope this helps someone.