As pH drops below 7, the shells will dissolve at an increasing rate. I’ve found that my Ramshorns are fine at around pH 6.5, with sufficient food. A calcium-rich food source can offset this. Your tank may not be supplying enough food.
Feed them with a calcium-rich food, once a week is fine. They get most of the calcium, for their shells, from feeding. There may not be enough periphyton, excess food, detritus, etc. for them. The problem becomes trying to prevent the fish from eating the food. So, what I do is to use a snail trap, which is nothing more than a feeder, of this type: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
. For food, I use this: https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
, but any similar food that is high in calcium would work. Some people use blanched vegetables such as spinach, but I've found these to make a mess and are hard to control.
Also, has anything been introduced that contains a surge in copper (water pipes or medicine)?
As far as re-conditioning RO water goes, the Equilibrium is a good, simple, choice unless you want to get more involved in mixing the components yourself. It does not address KH, though, and KH and CO2 are the only way to adjust pH. As CO2 increases, pH drops. To raise pH from a fixed CO2 level, you must raise KH. First, set your CO2 target and, as @pauld738
mentioned, try using the 1-point pH drop method, but be careful to ensure that the dKH reading is the same in the tank as it is in the sample water. When doing this it also helps to increase the accuracy of your dKH and pH readings. For dKH (assuming an API-type test), use 25ml tank water and divide the results by 5. For pH, a pen is much better than a reagent kit.