with 13 clutches i would be culling about 11 of them, because that many clutches is going to yield many many snail babies. each clutch should hatch out 50 to 100 snails. you don't want to have that many baby snails......... lol!
to cull clutches, just toss them in a ziploc and put it in the freezer for a while. once the clutches are good and frozen, toss them in the garbage. the freezing is to ensure the baby snails inside the eggs are dead and won't hatch out.
i use a teaspoon to get clutches off the glass. yeah, it damages the backside of the clutch a bit, but with the number of baby snails coming out of each clutch, a bit of damage isn't a big deal. the teaspoon seems to have just the right curve for me, to easily get the clutch off the glass.