I have 3 adult and 6 baby mystery snails in my snail only tank. I use coral as substrate and add calcium carbonate when i do water em changes about every week. I have diatoms that are beginning to be under control in last couple days. I use tap water that has been tested and no chlorine in it with prime conditioner when i do change water. Some of my snails shells are looking bad with holes and imperfections and some have perfect shells. I don't want them to be suffering. What am i doing wrong? Ph-7.6-7.8, amonia is now at zero, nitrite at zero, nitrates stay below 40.