I just started using Alum powder, because I read about it on this site. Go to the spice isle of your local grocery store to find it. Use 1 Tbsp per gallon of water. I tested this on pond, ramshorms, & MTS and they all died in 2-3 hours. Soak plants for 2 days to kill the snail eggs. Worked well on the Crypt wendtii & Hydrocotyle 'japan' I tried. The plants seem unharmed.
So you were able to soak the plants for 2 days without any ill effects?