Definitely overthinking, but, if we don't think we do stupid stuff and sometimes stupidity hurts
doesn't affect the ammonia that feeds the bio-bacteria.
The only thing that I can think in its current placement is that perhaps the water clarity flowing thru it isn't as clean/clear as it would be in the outflow of a canister filter. Water clarity is important, it allows the UVS to be more effective, any suspended matter blocks the UV
light from reaching unwanted guest. Thats why I put a nice pre-filter on the PH
filters are able to kill the following;
free floating algae
free floating parasitic organisms
free floating bacteria
filters are NOT able to kill
algae that is stuck to the glass, fish, rocks, plants
parasites your fish are already infected with
bacteria your fish are already infected with
Per the above, UV
filters are wonderful tools for free floating algae blooms and keeping the tank safe from parasites and bacteria if the fish are not already infected with them. A good example is the parasite ICH. When ICH is new born and free swimming the UV
filter will kill them and keep the tank healthy. Once the ICH have completed their free swimming life cycle and attach themselves to the fish, the UV
filter can't help further other then to kill baby ICH.
Like I said, "Nothing too good for my Oscar!".