Liquid chlorine. IF your CYA is 50, you do not want to add anymore. How did you get those test results?
Order a TF-100
The only other real option for a test kit is a Taylor K-2006-C. Be careful comparing prices because the K-2006 comes in sizes, designated by a letter. The basic K-2006 has .75oz bottles. You need to get the K-2006-C to get the larger bottles that you want.
I also have the Speedstir. It makes testing much easier.
When water is not properly sanitized organic contaminants begin to multiply. Low levels of chlorine are exhausted rapidly and is your first clue that organics are established and lurking.
You are unable to see low populations in your water. As the algae reproduces in ideal conditions you may notice "cloudy water" but even this is subtle and easily missed. When the infestation explodes the water takes on the green hue that often is the first clue for the negligent pool owner. Population dynamics are fascinating. Proper sanitation is all about prevention.