Doing 100% RO will cause wild variations in your water chemistry, since as the RO water hits the coral, or anything else in the tank for that matter, it will start to leach like crazy. Your KH and GH will likely be flying all over the place. Not a healthy scenario for plants or fish. Better to just remove the substrate, and if you really need to decrease hardness, use a layer of peat or add peat to the filtration. I say this as someone who's tap water is nearly RO - with near 0 KH and GH. Keeping tank parameters stable when doing water changes becomes a challenge, even with just a sand substrate that leaches very little KH and GH over time, I've lost more than a few fish due to this issue. In the tank with the sand substrate I don't dare change more than 25% of the water at any given time, because adding the 0 KH water to the 3 or 4 degree KH tank, causes major fluctuations when too much water is changed.
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