7.5 is rather low...significantly low, and can be the reason for mortality porblems. I'd suspect that before I'd blame the Coralbanded Shrimp for the snail deaths. Getting a second Coralbanded Shrimp and having that in the same tank is definitely not a good idea.
15% water changes every three weeks isn't all that frequent and that might be something you should do more often...at least every two weeks.
What's the pH of the source water that you use for your water changes?
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I'd keep the whole ocean if my den were big enough
Current Tank Info: 120 gallon reef with 210 lbs. of live rock, Aqua-C EV180 Skimmer, Aquactinic double 250W MH with blue plus t5 support; 58 gallon freshwater planted tank using CO2 and T5s; 30-gallon cube with a few fancy goldfish; and a 110 gallon FOWLR
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