What is he dropping the tank and checking for? Water...How are you going to see water looking into a black plastic tank? Thats a lot of work when all he needs to do is drain his water separator weekly. Drain it into a glass jar leave it sit somewhere for a day and look for a lair of water in the bottom of the jar. If your anal you can drop the tank, but thats a little over kill.
How do you feel about a sump? You could install a sump then you would know there is little to no water left in it. Or you could run something like Power Service 911 they have alcohol in them normally to help deal with water better. You just don't want to run them often since they are hard on injectors also it might not be such a good idea if there was a lot of water in your fuel tank.
The Fass did its job, and so did the diesel service. Read up on fuel additives. They help water separate out of fuel so the water separator can catch it. The problem was YOU never drained the separator.