It's such a pain to shake the #2 nitrate test bottle (API water test kit). Shake it, pound it, repeat...for 3 or more minutes. And it can still clog up or give you an inaccurate test. Here's an easy way to fix that.
1) Remove the dropper nozzle by wiggling it out very carefully with pliers. Yes, it pops right out. How do you think they get the stuff in there? ;-}
2) Drop in a pea-sized bead or pebble. Glass is best; make sure it's clean.
3) Pop the nozzle back in.
You now have a rattle-can dropper bottle which will mix in less than a minute, and be more accurate as well.