Help with parameter

So I have been trying to raise my calcium and kh levels in my 55g fish, snail, and planted tank. I have actually tried a bunch of things. I was doing water changes every two weeks. I have now changed it to once a week to try and help replenish the calcium mostly for snails and plants are doing good, snails not so much. Fish are fine as well. I also use flourish and flourish Excell and a water conditioner for water changes. No other chemicals or special treatments.

So I have tried adding crushed coral to my filters and no my kh went from 20ppm to like 40ppm and refuses to move. Calcium is 20ppm and sometimes but rarely 40ppm as well. My GH is over the 300ppm and ph is 7.4 to 7.6.

I recently added one of those Wonder shells (Giant) and the snails seem to be more lively now and everyone still doing good but parameters just seem to stay in place.

The reason I wanted to raise levels was my mystery snails shells are showing some discoloration which it seems is related to calcium.

Is there something I'm doing wrong? Should I be doing something a different way? I expected levels to rise slowly but it's been a few weeks now. Also I use liquid testing kits to get numbers.