I have lost 8 of my trapdoor snails since I purchased them 2 months ago. They don't come out to eat, they stay in the same place & don't move. I have a 75 gal. with oscars, dempseys & pearls. All fish are great. I have 2 Aqueon 55's on the back, a bubble wand on the bottom, I use well water, it is considered hard, but all other water test levels are perfect. I do 60% water changes weekly. They have plenty to eat. They are not active. Please help with any advice.