If you buy pool chemicals the cheapest place to buy them i have found is on Choafo west rd same side as Wat Chalong and about 3oo to 400 meters west
50KG of 90% granulated chlorine 3800 baht, 50KG of aluminum sulfate 500 baht, 50KG of soda ash 850 baht, same price the last 2 times i have bought it, it may have gone up slightly in price but its still much cheaper than elsewhere, i have seen pool servicing companies buying their supplies from the same business