How much room do you have? A larger volume of water will help with stability- both in the case of something dead and salinity. I suppose it will all depend on what you want in your sump. Adding live rock? Cheato? A deep sand bed? Bioballs? internal or external skimmer? Filter socks? Internal return pump or external? An automatic top off? Any reactors? Do you have the space for a 220 Gal sump on your tank
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Yes, a million questions (my whole role on the board is comprised of increasing the total number of questions rather than just answer them.) I will see if I can not find an article on a sump and sump design- but for now I would say look at some of the build threads.