I think with the fishy food or any food ... it's the ash or magnesium content. I know this can cause crystals to form in the urine... like kidney stones. Not sure about a URI. I also understand it's more of an issue with males.
Thanks for the great tips, everyone! O continues to do well and acts as if he'd never been sick at all (thank goodness!). I served him and Clementine some spring water (which we buy by the gallon), and they LOVED it! Both have been lapping away, seemingly, all day--we even had a skirmish by their waterer, which is a first.
I'm definitely taking your advice re. the fountains. I'm not sure what our PetSmart and PetCo will have in stock, but I've jotted down the name of the fountains recommended. I'm going to be cleaning its little motor, for sure--after all, if O and Clem take to the fountain, I don't want it breaking down on them!
Thanks again--O and Clem send their very warmest purrs and thanks!