Good Day, everyone. I have been using the TFP method now for a month with my new spa and everything is magnificent, many thanks. I will be in need of PH drops and was wondering if they are pretty much universal? I can obtain them locally at a reputable hardware or spa retailer. If this is a correct assumption, do I have to be sure they are the "5 drops at a time" type? I seem to remember seeing/using some that were 3 drops per test decades ago. Your input is greatly appreciated.