2 Filters or one?

And, at least around here, you can get store credit without a receipt on items stocked at petsmart. I just returned a bottle of stress zyme and a bottle of stress coat which I'd lost the receipts for; Funded my incandescent hood w/ 13w daylight spiral cfl's :P

Edit; Worth noting that it doesn't matter if the product is used(including chemicals, food, etc), my bottles were partly empty, and a while ago I returned a topfin filter that had been used about 2 weeks.
 
i have a whisper 40 on my 20 gallon high. works fine and does a fantastic job. id go with 1 whisper 40 myself.
 
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