They kill pretty much whatever goes through them, so suspended al;gae spores or Green water is controlled.
Bacteria , cyst, disease propagules will also die.
As far as causing any noted effect on nutrients, trace iron dosing, this has never once been demonstrated or any noticable effect in anyones aquarium to date.
It's a very easy thing to test as well.
Turn UV on, say for 3 weeks, note plant health.
Turn UV off for the 3 weeks, note plant health.
Dose as normal.
Repeat this several times, say 6X.
Then you can be fairly sure what you see is more than likely to be due to UV effects on the traces.
But no one has.................nor seen any differences they can honestly attribute to solely the effect of UV on nutrients.
So while there's a theory...........it's not well supported by any observations by aquarist.
Perhaps it does break the chelated bond.............some spectra of UV dose do this, better than others.........the ones used for germicidal effects are typically different than those used for some chemical bond breaking.
Also, different chelators for Fe will have different bond strengths and thus also have different abilities to maintain chelation.
Fe typically does not last long in solution anyways, fortunately we do not need a lot, as far as I know, the rest of the nutrients are relatively unaffected.
So basically there is a lot of talk, discussion about the theory, but not one person has done much about verification or has there been any noted correlation between UV use and low Fe in a planted aquarium.
Too much talk, not enough good test.
I've never been able to see any differences to date.
And if the bond is broken, can the chelator rechelate the Fe?
Regards,
Tom Barr