What are good chemicals to use for a 18′ above ground intex pool?

You need to add a ‘sanitizer stabilizer’. The active ingredient is cyanuric acid. I’m not a chemist but the best I can describe it is like sunscreen for your chlorine. Chlorine will quickly evaporate in the sun and this will stop it. You can pick this up an most places that sell pool supplies. The good part is you only need to add it when you fill the pool and maybe a small amount as water is lost and replaced from splash out.

Your chlorine demand should drop dramatically.

It takes a couple days for this stuff to disintegrate

I hope that helps.

Winterizing Your Above Ground Pool