I've been a koi keeper for many years, the API Freshwater Master Test kit that Big Al suggested is a reliable and accurate one that I have used for 15 years. Unless your local aquarium or pond shop has a lot of stock turnover, buy from one of the big online stores to be sure to get a "fresh" kit - not one that may be near to out of date. The API tests are good for 3 years from the date of manufacture according to Aquarium Pharma. The date is stamped on the bottle.
Unless someone like you cares a whole awful lot, nothing is going to get better. It's not.-- Dr. Seuss, The Lorax
Simplicity of character is the natural result of profound thought~fortune cookie