Think about a river and the different make up of chemicals in it as it rains, or snow in the mountains melt. They are used to some changing attributes of water (hardness, pH, temperature...).
Now in reef setups then you need to keep everything steady (hence RO water) as in a reef the make up of the water rarely changes, but the vast majority of FW fish can handle the minor changes in tap water.
And I for one don't like the expense of setting up a system to remove everything and then replacing some of it back. Not to mention the horrendous wastage of water from RO. What is it now? 3 gallons waste for 1 gallon good? Anyone on a water meter is going to have trouble finding their water bill has gone up 4 fold just so they don't use de-chlor.
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