Maybe crush a pill in RO and test the pH just in case. Also check the inactive ingredients (although I'm sure you have). The vitamin C tabs I have at home contain calcium phosphate and silica as inactive ingredients.
Slightly off topic, but I'm curious what did you use to test the silicates? Ive been wondering if there was a test kit, but Ive never seen one. Where can I buy one?
Yup salifert silicate test. And anyone trying the same please stop immediately. my silicates just jumped to 2 ppm. I just pulled out the remaining tab and did a 45% water change. and will be doing 1 each day for the next 2 days. The additives in the pills are dangerous. started making my feather dusters unhappy. I picked up some ascorbic acid today from the store and will continue trials.(ascorbic acid is pure vitamin C)
Added 2000 mg of ascorbic acid. vit c levels are sitting at about 10 ppm. Zoas are looking good. Note: add the ascorbic acid slow or you could spike drop your ph.