There are several ways to approach this kind of business. I like the fixed monthly price whatever it takes to keep the water in balance approach, but many companies go with the fixed cost for a visit plus detailed charges for anything even remotely non-standard approach. By charging one price regardless, or even offering a discount when there are problems, you get paid when the pool is looking good. If you know what you are doing that can make for very happy customers. Otherwise you are in the getting paid extra when something goes wrong business, which can lead to conflicts. Pricing varies quite a bit from place to place. Call around to a couple of companies in your area and get price quotes to see what the going rate in your area is.