That is true. And if I understand it right that is because there is a higher chance that water will contain phosphates and nitrates. Phosphates can be removed using media like GFO (granular ferric oxide). I have had good success.
The things algae need enter the tank in many ways, and your tap water is only one. Such as excess food, fish waste, decaying things in there that are not removed due to insufficient filtration. If you have good filtration, good skimming, feed appropriately, and your tank is overall clean and nutrient free, you may be able to get away with tap water containing less than perfect nutrient levels without big algae outbreaks. If your riding the line to begin with, tap water could def push it over the edge.
For me, I'd def switch, since where you get your water is so easily under your control, id get good RO/DI water so you have some room for error in a different, harder to control area of the hobby.
I have run GFO since day one, my tank is pretty lightly stocked for it's size and I feed moderately at most, and I used store bought boxed ocean water. Never really had an algae problem. I only recently employed a heavy duty UV for preventative for parasites and such.
