Why did my malaysian trumpet snails all die?

i bought 20 MTSnails and they were doing fine in a one gallon tank i used to quarantine them, they even bred in the tank and i had 12 tiny babies i kept in the 1G tank. i added the parents to the main tank which was equal temperature, but had more waste in it due to the fish, they started cleaning the tank walls straight away and burrowed in the gravel, i decided to leave the babies in the small tank and carried on with water changes and fed them 1/4 of an algae pellet- advised feeding for snail only tanks. i carried on putting a full pellet in the main tank for the adults and the fish didnt try to eat it, the snails loved it, i reduced the full pellet down to 1/2 so they could eat uneaten food and algae and waste from the fish, which they did, however i suddenly lost them all. i cleaned the tank, moved the gravel and realised some of the shells were white and empty- dead. i found 3 which were alive and took them out. all the rest were dead. the babies i had were ok, and growing bigger, i put the three adults in the little tank and they became more active and ate food and seemed fine. (meanwhile all my fish were fine, the water i took to the pet shop tested fine and they told me the snails could have died from the journey- they were delivered to me by post (but this was weeks later?) anyway, a few days ago the babies died and 2 adults, i have 1 adult left but it wont come out of its shell- possibly dead. and 2 babies which are out of their shell, but not eating. i took my water for another test today and the shop have insisted the water is ok in terms of parameters and it could be possible it was a disease. im now worried my fish could be affected, maybe this disease takes its time to appear?