In my experience, as was said above, the UV sterilizer can be a good tool to have to deal with something, be it bacterial, parasitic or algae. Therefore, having one that can be added to the ecosystem when needed is important. In other words, you don't want to run one 24/7.
To accomplish this, many people use a
hang-on or an
internal UV sterilizer. Of course, if you plan on using it relatively often, you can plumb one into the filter return but I suggest using a
T fitting with a valve to allow you to divert flow to it when needed and to control the flow.
I always recommend getting a more powerful one than you need. Most beginner UV sterilizers come with a 10 watt bulb. I would recommend going with a stronger 20-30 watt if possible. This gives you more flexibility and it sterilizes quicker. Most people use it for algae so you want faster flow through it so you need more power due to the lower contact time.
Lastly, you need to be able to control the flow through the UV sterilizer. The general rule I've read is between 3 and 4 times tank turnover as a starting point. If you are battling a parasite (like ich), then a slower flow is recommended as you need longer contact time. However, for fast growing algae, you want more turnover with enough power to kill it. By being able to adjust it with a valve, this gives you the luxury of tuning in the optimal flow for your situation.
On the impact it has on our fertilizers and the plants' ability to get nutrients, I have never experienced any negative effects. I guess if you run it continuously, you may have some issues. Running it intentionally for a period should be perfectly OK IMO. YMMV.
