I doubt that was the cause of the snail deaths. What is the tank temp and salinity? Your tank looks very new with virtually no algae for them to eat....they don't eat cyano
, which is the purple stuff you have. That's caused by a high nutrient level in the water. Being you have to use tap water, you may be fighting an uphill battle. You can try running a high quality carbon, such as BulkReefSupply's ROX 0.8, along with some GFO
. They will help pull out the nutrients but would probably have to be changed out quite often and it can get expensive.
You can get a good RO
unit off Ebay for right around $100.