Hello to you too. :-)
I haven't tested this or anything. But my impression (which could be wrong) is, that the water quality is best, when I have one of every filter for a tank. Of course only if I need so many filters to reach the required quality. I always set a filter first, a skimmer second and nitrate last. (Or if I have access to the one thing which is a combination of those three I use that if the tank is big enough or the occupants need very high quality.)
In very large tanks (I am playing the last campaign) I need more filters and I haven't been able to figure out any pattern as to what is best for a given tank. If there is such thing as "a best additional choice".

I used to swap the filters (while the game is paused of course) from other tanks to the tank which needs better water quality to see if it makes a difference if I add one more filter or skimmer or nitrate. Sometimes one would improve water quality more than the other two but I didn't see why. But I also didn't consider that maybe the occupants were the cause, e.g. if there is one that requires a very high quality that maybe the nitrate would give the biggest bonus in this case because the descrpition of it states it is specifically for reaching a very high quality or something like that.

I had this answered in another forum.

Apparently the best increment of water quality is gained from installing the filter with the lowest current representation in the tank. So if I have 90 points of filtering, 90 points of skimmer and 90 points of nitrate reactor, I will get a better quality bringing the UV filtering to 90 points, instead of increasing any of the others.

However, the newer techs are more expensive, so it falls to us to determine whether to install 1 high cost equipment to solve the problem, or install 2 low cost equipments and get the same result.

I think I have never used/tried the UV filter. Maybe I should...
Thank you. :-)