Just to give your all something more to talk about. If you have clear, or slightly clear, tank of water setting in the sun it will quite quickly start turning green. No matter how careful you are. But, a tank setting in the shade (like most RV tanks are) will not grow anything ; easily seen as green ; for a long time.
I know this because on the farm we have a off white colored 1500 gallon tank of water setting outside that we use for filling the farm sprayer. We can not leave that tank full for very long or it gets green and plugs up the filters on the sprayer. That tank is filled from a treated city water source.
My TT tank is sanitized once a year and sets under the bed. It never grows anything green in it. Yes, I can see it by lifting the bed and that happens fairly often.
