OMG Aquatics

http://aqpkg.com/Styro-Liners.html

Try adding some Seachem Pristine. There is likely decaying waste you're not seeing in your tank.

Blue Bolt Low Grade and Blue Dreams

GH is WAY too low. Neos should have a GH of 6 - 10. Find a way to lower temps as well.

Increase the height of the spraybar or lower water volume.

Did you add ammonia into the tank for cycling? Are you using any nitrifying bacteria supplements?

Looks like it has green fungus disease as well.

http://www.chelonia.org/articles/waterchemistry.htm https://www.discobee.com/blogs/news/17030569-dwarf-shrimp-water-parameters

That looks like inert substrate? If so, that's a problem.

Would be best to just net out all shrimps into a container and do 90-100% water change. If you have a large buildup of mulm, that may be why your pH got so low.

Using limestone for GH isn't a stable method of maintaining GH. Every time you do a water change, GH will drop and the limestone will slowly increase. Also doesn't limestone increase KH? You said your pH runs at 6.5 but could be lower. If you have a KH of 2, there's likely too much waste in your tank lowering the pH. Neocaridinas don't do well under 7.0 pH

Your GH is way too low. I recommend a GH between 6-8. It's highly likely that your shrimps are dying due to unsuccessful molt from the low GH and the assassin snails are just eating the dead ones. They also prefer a pH above 7. You would need a remineralizer to increase your GH.

NY

People believe that caridinas do not move as much which results in nutrient deficiency which results in lower survival rate. Neos tend to swim all over and explore which is why their survival rate is likely higher.

About 1.5 years in 3 tanks. All substrate will break down eventually but usually will likely take years.