sterilizers will kill any phytoplankton, zooplankton, parasites, and bacteria that pass through them, resulting in clean, clear, sterile water. The key phrase here is "that pass through them". Unless your pump is way overpowered, there is no guarantee that all, let's say... marine ich in the swimming stage... will ever pass through it. So, while it may reduce numbers of parasites, it will not kill them all. Period. Still, every little bit helps, right? This is what people like about them. Helps reduce the risk of disease outbreaks.
Now the other side. Also kills free floating micro algae, or phytoplankton, which is the first step in the food chain that leads up to zooplankton (will also kill them) such as the pods
that our fish love so much, and some fish even require.
So, will it help prevent ich? To an extent, yes.
Keep the water clearer? Yup!
Kill all the beneficial pelagic organisms and phyto? You betcha! So, it's a tradeoff. personally, it's not worth it to me to deprive my pods
of food just to make me feel better about the risk of ich outbreaks.