Snails and crabs dead, shrimp and corals ok?

[JCareyETexas]

Wondering if somebody can think of something I've yet to check. First my turbo snails started to die off after being in tank for 8+ months, next to go were the astrea snails, after that a few catalina snails that picked up lasted a couple weeks then kicked it. Now I see no movement in regards to my hermit crabs, they theoretically could all be snacking away in my rock but it is unusual for me not to be seeing a couple of them at some time or another.
This seems to coincide with a hair algae outbreak that I am battling with and am unsure if the events are related.
I can understand that hair algae may strip film algae of nutrients needed to grow and therefore starve the snails but I wouldn't figure that would hurt the hermits.
I cannot confirm the death of all hermits but have found a couple empty shells that have me worried.
I assume somebody is going to jump in and say copper. My RO water reads 13 on TDS meter (I was told that is ok, what say you?). If it were copper wouldn't my corals and shrimp be affected as well?
Any ideas?

[Fish_King_25]

You didn't give much mention to what it is you have checked already, so I will start with the basic question of what are your parameters right now? I am going to jump out and say either nitrates or phosphates as a first impression, but if you could provide a list of paremeters with there corresponding values, that would help everyone with giving you some insight to your issue.

[JCareyETexas]

SG: 1.026
NO3: < 5ppm
NO2: 0
Ammonia: 0
Phosphate: 0.12
Temp swing: 74 - 78/79

[Fish_King_25]

Well your parameters seem to be pretty close to in order, you do have a registered amount of phosphates, and of course anything except 0 could potentially be an issue, but the small amount of phosphates aren't going to kill off that many snails IMO.

I can't give you any insight on a copper issue, or an issue with the amount of TDS you have in your water, because this is an area I am unexperienced with.