If your seltzer recipe includes fruit, pectic enzymes works wonders. It works best if added before fermentation but will still work post. It will not harm your yeast.
I didn't have to use a fining agent at all in my batch. After I bottled and put a few bottles in the fridge for a couple days, mine was VERY clear. Like spring water clear.
I used a flavor extract for flavoring however, not a puree.