your reservoir size should be at least equal to the size of the flood table or grow cell. I have one e n b setup that uses a 13 gallon flood table w/ an 18 gal. rubbermaid res and it's perfect. I'd go with a 100 gph pump per 10 gallons of res size in my 18 gallon res I use the eco-plus 185 gph pump and it's just fine. If your gonna run a 40 gal res use at least a 400 gph pump... just my $.02

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