Bromley, I might have used the wrong word when I said "cling". It is not a static cling, but it hangs on by suction cup. It is made by Seachem. Here is the link to the exact one that I bought at a local Petsmart. Seachem makes Prime, so I think most people trust that brand. (I'm still new to the fish world, so I have no experience to go by)
Seachem Ammonia Alert
Edit: Found a review about it on the combopack on the website. Not sure how accurate this info is..
"I have the ammonia alert tester, and so far I've been very impressed with how well it works. HOWEVER, I've seen people give it a bad reputation due to the fact that they don't understand what it does.
As it states right on the package, the reader measures the amount of FREE ammonia in the water, not TOTAL ammonia. Free ammonia the kind that is toxic to fish, as opposed to ionic ammonia. Ionic ammonia is a non-toxic form of ammonia, and together the two make up total ammonia.
Furthermore, the ammonia reader is NOT designed to be a stand-alone test of ammonia levels, but rather a supplementary reader. In other words, you DO need to continue test your water on a regular basis for total ammonia in addition to anything else.
However, that isn't to say that the reader is useless. In fact, I've found that it makes a great first-warning system should an ammonia spike occur. In addition, as it also states on the package, a new reader may take a few days before it starts working correctly, although mine seemed to work right out of the package. I've also been very careful not to touch the sensor dot, so not to influence it with any chemicals from my hands.
All in all, it does its job well, and should be viewed as a first line of defense in the case of a spike in free ammonia, rather than a fix-all measure of total ammonia, as that's not what it tests for. Furthermore, I will probably be getting the pH reader as well once I get my new tank, and I've already recommended the ammonia reader to my roommate."
