Dear Ari,

My understanding is that good or bad bacterial get killed as they pass the UV light.
So, only those that is water bourn and gets unlucky enough to go pass the UV unit get killed.
Those in the filter bed, under-gravel or canister will not be effected.
I suppose, it is the good bacterial in the fiter bed ( again under-gravel or canister or sump ) that is doing the significnat part of the work converting ammonia to nitrite and to nitrate.
As to the bad bacterial in the filter bed, no harm to the fish directly if they stay in the filter bed and the fish do no stay in the filter bed.
If they get out of the filter bed, depends on their luck lah.
If they gets stuck to the fish before they go pass the UV filter, the bacterial win.
If they go pass the UV filter before they get stuck to the fish, the UV filter win and the fish gets lucky.

I suppose this is only theory.
No way to proof that it works.
Just have to trust the manufacturer's recommendation.

In my case, bought 2 China made unit with no operation instruction at all.
Like to hear form other user who has been successful with its use.

Cheers!

BT Lee