I did have a rock that I had tied riccia fluitans on; I took this out of the tank tonight...
Which tank was this in? Rocks can supply a lot of different minerals. Try putting this rock in a jar or RO water. Test it after a day or so, then several days later. GH, KH, pH.
Do the same tests on the spring water you have been using. The companies that bottle the water have no requirements about what goes into the water, and can change their recipe whenever they want. Much better for you to not use spring water for aquariums.
What is in your tap water that you do not use it?
When you start with RO water you need to add minerals calcium and magnesium to suit the fish. What kinds of fish are in each tank? If they are soft water fish then your goal of 3 degrees or a bit more is good. If you have hard water fish then you want the GH to be higher.
Then add potassium bicarbonate or sodium bicarbonate (baking soda) to raise the KH to something pretty close to the GH. The goal here is to make the pH in the right range for the fish and make it more stable. Soft water fish generally prefer a pH on the acidic side of neutral to about neutral. Hard water fish generally prefer more alkaline pH.
If you are keeping black water fish then add some peat moss to the prep to add the organic acids these fish thrive in.
To make the change go slow.
I would still use some of the spring water for your first few water changes. For example, make a mix that is 25% RO with the right minerals added + 75% spring water. Do 1-2 water changes of about 25% a few days apart with this. Then do the same thing with some water that is 50/50. 3-4 water changes over a period of 2 weeks. Then a few more with water that is 25% spring water + 75% RO and minerals. After that you can use the RO and minerals with no added spring water.
This sort of change is very gradual, might take a month, but will give the fish plenty of chance to get used to the new mineral levels, and adjust their metabolism.