Clean your filter frequently. It damages cartridges to have a big pump run against them when they are stopped up.

Ideally, you should have at least two cartridges so you can rotate them.

Longer term, you'll need the K-2006 kit + an OTO/phenol red kit (like the HTH 6-way). Normally, you'll only use the OTO/phenol red, but you have to have the K2006 and use it periodically, to keep things on track.

It is much, MUCH easier to keep a pool on track, than it is to clean-up when it 'goes off the rails'. The upside is, doing it yourself -- the RIGHT way -- is much, much cheaper than using pool stores, so you'll fairly quickly save-back the money spent on test kits.