Good to know. With the many reviews I've read for AIO's, and other liquid cooling, I hadn't really come across much mention of testers for these setups. Having worked on cars, and seen some nightmare, I should've known that things like this exist but I'd never heard of them until now. I only knew I didn't want to experience leaks where electricity is concerned. It's what's held me back from doing any liquid cooling so far. It'd be nice if someone did some comparative reviews of these kinds of products.
These have been around for a while by other brands. They seem overpriced for what they are unless you don't have a hardware store nearby. I bought one of these: https://www.homedepot.com/p/DANCO-0-15-psi-1-10-Increment-Gas-Test-Gauge-94352/100180536 last week with a 50 cent 3/4" to 1/2" barb adapter and made my own for a total of $11. Of course, that assumes you already have a bicycle pump or other pump that you can use to pressurize your loop. I also already had the compression fitting and rotary fitting as well as thread seal (plumber's) tape lying around. There is some giant thread around on overclock.net somewhere where they talk about how to build your own.
My homemade tool:
It could be better (a shut off valve in between the gauge and the schrader valve would be nice but isn't a big deal), but it worked well enough for me to find the leak in my loop that was too minor for water to leak out but enough for air to get in.