Ocean forest is pretty heavy in nutrients no? Maybe the lighter soil was better to start in. At any rate, besides the tips the plant is green and moderately healthy looking, so I'd go with the safer rout of sticking with aged tapwater until you notice uniform healthy new growth.
you tap water is fine no need in buying water, if the tap water ph is to high or low that can be a prob though but you plant looks fine besides the burn so i think the ph is ok. its most likely from the soil being to hot for the small plant and it got a nute burn it happins alot with FFOF. your plant will be fine just continue what you are doing.
has it spread at all