As a general rule, chams don't recognize standing water. They only react to misting or dripping water. The only mellers i have seen in person, had a dripper and holy schnikees were they all HUGE!
From what I've heard, Mellers need LOTS of water! I'd say a dripper is a must, but most owners I've heard also give them long misting sessions (like 20 min) a couple times a day.