Waybeuno -

For short cleaning (short of stripping the tank), I use boiling water. When doing a full clean, we use a mild bleach solution and then rinse with boiling water. This is for plastic branches and decorations. I second Kurt on the cooking of actual wood.

As for my personal experience with color, I again second Kurt. My whites tend to change color based on location. The primary thing to be concerned about isn't necessarily the color but a change in the normal patterns of color. Color change combined with other deviations in behavior such as loss of appetite, decrease in activity, etc., can indicate sickness. In other words, get to know you're little guy. As long as other behaviors are remaining the same (is he still eating well, still as active as before?), than you have nothing to worry about!

-IgbyKibbits