You need 5/8 internal diameter hose, be careful at home depot they have 5/8 hose with 3/4 and 7/8 external diameter. The 7/8 won't kink as easy but is a lot harder to bend over the top of your tank and the compression fittings on the aquastop valve will get stuck on them, it will work but won't look as nice and you won't be able to change it without buying a new valve. I'm pretty sure the stock hosing is onlyy a couple of dollars more, I've seen it on amazon.com and many lfs.