Yet another slight variation - FWIW:
I use distilled water, top off the tank nightly, drain and wash the tank weekly. Went for almost a full year, and then found mold in the inside corners of my tank. (replaced that puppy real quick)
I have since been draining and drying it daily. But I am also wondering if perhaps the mold was because of a bad bottle of distilled water and I am seriously thinking to go back to my old routine.
No, the mould was from not drying your tank regularly - unless you distil your own water and rigorously control it, you can never trust the purity of what you get, but either way, what you did was just plain unwise. As I said, even the purest distilled water will develop into a growth medium after a few days or sooner. So leaving the water in and topping up is not the best choice to take. It may work for you, but I would bet anyone who took a swab of the water or the tank after a week would find things that would shock you.