Good to hear you are learning better ways to keep your turtle and although it feels terrible when these types of incidents happen, it wasn't the first time and it won't be the last time. Since you are doing your best now, keep learning, and make a great home for your pets.
Are you familiar with the nitrification cycle?
Check this out and it will help give you many true and useful tools to keep your aquariums better! There are links for other helpful articles as well.
The big thing is you need to know that these snails have been subjected to many things recently, travel to the fish store and changing again to new water system, then to you and more changes and equipment issues.
Their bodies can be damaged and in some cases they can recover.
Once I had accidentally injured a snail hit with a salt mixture and the poor guy for sure got burned (I didn't see him in the spot I dumped in the water
) it took a month of not really coming out of the shell and thinking he was dead. But he did recover and live a long life after that.
Keeping the water in as pure /excellent parameters as possible is very important.
ammonia, nitrite both 0 and nitrate 10-20 ppm
This helps make sure the above are not injuring your pets.
Do you have a liquid test kit?
Do water changes each day for awhile after getting the parameters at a safe number. Once at a safe level, then do a 25% pwc
daily for a week and see how it is going. If your numbers are still 0 for ammonia and nitrIte and in safe zone for nitrAte, then skip a day and again test and do your 25% pwc
Always of course, using temperature matched water which is the same and treat for any chlorine, chloramine and heavy metals.