Does UV kill everything

[Delphinus]

In short, no. UV can only affect that which passes through it, so with certainty it can not do anything to help manage flatworms or aiptasia. Even for things like cryptocaryon it can only assist in neutralizing the free-swimming stage of their life cycle.

For the most part UV will make a difference for water clarity as it helps break down the compounds which tend to make the water yellow (similar to ozone). For pathogenic control purposes such as for ich control you need a large UV. For example, 50w or up. The smaller units like 8w or 15w or in that ballpark are intended for clarifying water more so than pathogenic control.