I still have no idea why you would need a sandtrap. If you don't have a sand problem, no sandtrap is needed.
Cyanuric Acid is a Stabilizer and is used in Swimming Pools. I have to add it to my pool occasionally since I quit using the three inch tablets which use a chemical that I won't attempt the spelling of here. The three inch tablets have the acid in them, I now use a Chlorine Generator to keep the pool disinfected and therefore have to add Cyanuric Acid.
If you live in a Farming Community, I would be more worried about Fecal Bacteria and Nitrate/Nitrites in your water. There is the possibility of other chemicals that could be in your water due to the proximity to Farms that can't be tested for at a Swimming Pool Company. That's what the Labs are for.