I buy HammerGel in the big bottles. It's a heck of a lot cheaper that way and easier to put into the flask. I mix and match flavors for variety.

I do add just a bit water to the flask to make flow more easily but I don't to the large bottle. You have to keep it refrigerated and I wouldn't want to possibly add any contamination to something that needs to keep for a long time.

By the way, the valves on the FuelBelt brand flasks are much softer rubber and easier on the teeth.