Just reading this thread in general there are a few things points worth pointing out. My experience is approx. 15 years as hobby, few years a serious breeder both marine and freshwater so just sharing the mistakes I have already made for your benefit.
Right 1st of API test kits they are crap stop suggesting them they are 1 up from a test strip and very inaccurate and are sold to people who want quick and easy. Nice fish tanks are not quick and easy they are huge learning curve at 1st
If you want test kits that are fair price and as accurate as your going to get this side of £300 you will want to buy
Sailferts there about £10 each and you need 3 or 4 for freshwater. Lots of people say my API is fine, take your stats with an API pay a lab £100 for analysis.
when your buying fish at £100 each you wont be trusting API kits for tests for sure.
Join a proper fish forum get to know people and get decent advise. I have been in a lot fish shops over the years some of which there knowledge is awesome BUT and a big but if they told you the truth 100% they would close down they need to turn a profit selling there products fish are not money makers in the trade(In general)
last wall of text is test your tap water, after standing for 24hr the chemistry in the water will have changed. Sometimes not much sometimes a huge PH or ammonia swing, you might have really high nitrates up to the legal limit. HTH