drain em, open em, wipe em
We drain, either through the previously mentioned methods or just when we run out, (showering is a really good way to accomplish this), then my wife with her small arms reaches inside the tanks and wipes them out. We liveaboard and so drink the water in our tanks. All our water is either directly collected rain or pumped from tanks filled by the rain. We filter it going in if from a tank. We still end up with some growth and sludge. We only put bleach in if we are leaving the boat for any length of time.