Many trusted sources say the problem with reef tank UV systems is that unscrupulous vendors sell them WAY undersized in terms of intensity and dwell time to be of any use at all, unless you get a big expensive system.
This looks like a perfect example of completely useless due to being underpowered.
UV effectiveness is fairly simple to figure out. Intensity + exposure time and a chart that you can find multiple places online will tell you what you need as far as the unit itself and GPH. These cheapies are at best a clarifier, but won't do what you want.