Can someone help me figure out what killed my Mystery Snail?

[ReeseADHD]

Hey, so today I was feeding blanched broccoli and carrots and I noticed that one of my Mystery Snails wasn't coming out to eat. I didn't think much of it because snails are pretty derpy and they tend to do their own thing. After a few hours I decided to sit down and try to find where the missing snail was in the tank. I found it laying upside down behind a rock. It's trap was open and it was beyond dead. If I had to guess it's been dead for at least 1-2 days. I really love my snails and they're my favorite inhabitants in my aquarium at the moment and I really don't want my other ones to die. I'll list details about my set up below, if anyone with more experience could help me figure it out I'd be greatly appreciative. 

*Edit* - After I discarded the snails body I examined the shell and I found a tiny hole in it. The area by the hole was brittle, I was able to break parts of it off by applying light pressure with my finger nail.

Set up:

44 gallons

Sponge fiter

Fluval 207 (With intake sponge)

Heavily planted 

Heavy use of rocks - 20lbs slate and quartz 

Half the tank has sand substrate & the other half is Fluval stratum 

Moderate amount of brown fluffy algae (diatoms?)

Heater set to 75f - Fluval E100

Spider wood - 13 inch

 

Stocking level:

4 mystery snails (now there's 3)

9 amano shrimp (I had 10 but one died shortly after adding them - I don't think that's related but it might be)

 

Other Info/Parameters:

I feed every 2-3 days with Hikari Crab Cuisine or Xtreme Shrimpee pellets. 

The tank is cycled and has been up and running for a month.

Ammonia - 0 ppm

Nitrites - 0 ppm

Nitrates - 25 ppm

pH - 7.4 - 7.6

GH - 200-250 ppm

KH - 40 ppm

Edited April 23, 2022 by Lucid_Reese
Adding more information

[Guppysnail]

I’m so sorry for your loss. Once there is a shell breach it allows bacteria into more sensitive parts of the snail. Unless repaired quickly they do not make it long. I’ve had ones drop and cause damage I did not catch in time. Age also plays a roll. Unfortunately these beautiful creatures have a very short lifespan. 

[Colu]

Your kH is on the lower said I would add some crushed coral to raise your kH and feed your snails something like shrimp king snail stixx with added calcium as  brittle shell can be a sign of mineral deficiency

[Brandon p]

@ colo is correct I have almost no Kh in my tanks. I leave could she’s on the tanks and add Ca with magic shell. I have tons of snails the that I sell to the pets t store for feels of others fish