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The prefilters you'll find at walmart are intended for drinking water RO systems. Think 12 gpd membranes and the like. Often the filter specs are not even provided on the filters. When the are, they are not of a quality suitable for the high gpd systems used in this hobby. I'll give you a dollar if you can buy a carbon block with a chlorine capacity specified on it at walmart. HD and Lowes sell the same sort of stuff - and they are more expensive than people in this hobby can buy at from RODI specialty stores that cater to this hobby. Not sure what you mean by "padded filters."
Russ