I recently got back from a somewhat extended leave away from my fish (4 days total) and I quickly decided to test the water in my goldfish tank because I know it is technically overstocked.
I tested it with my liquid kit and my strip kit.
The results are, if you hadn't guessed, the strips are horrendously inaccurate. I tested nitrITES and nitrATES because it is an established tank.
**Disclaimer** Please don't complain over the semi-dangerousness of the following results, I immediately did a large water change and re-tested for safety. My goldies are fine, I almost never do this, hence why I thought up the experiment the second I got home**
**Disclaimer #2** I change my goldfish water 70% every other day and 20% everyday, just to clear that up. I test every other day because I am fully aware that their tank is too small for them. I think I check on them more than I check on my own health**
As you can see, for some reason the strip didn't turn the deeper purple colour as it claims to do so when there is 80ppm of Nitrates in your water, it kind of just turns a little bit pinkish than the "safe" zone which is under 40ppm.
If you value the safety and health of your fish, I definitely recommend saving up some money to purchase this liquid test kit. The "master kit" (the one in the hand sized box) is very good value. My petsmart has it for $35 ish and it regularly goes on sale. Since you are adding literal DROPS of this stuff into little test tubes, it should last you a long time.
1 drop is approximately 0.05mL and each bottle is about 37mL, which is approximately 740 drops per bottle. Given that you don't need to test it everyday (unless in my situation), this kit can easily last you anywhere around the 1 year mark, depending on your usage.
Paying $35 to almost guarantee the safety of my fish for 1 year sounds like a pretty good deal :P