What’s on top of my water tank?

[Breakin Away]

My forepeak water tank has a white cap on top with a bunch of ridges around the wdges. (I'd post a pic if I had it.) I've been unable to twist it off. Is this a removable inspection port? Is there a special tool for it?

[John Langford]

It is an inspection port. Most owners use Teflon tape to make it leak proof. Try one of those big plumbers wrenches but be gentle.