Tap water often contains high levels of minerals, and over time they precipitate in the dirt the plant grows in. Mostly calcium, which is useless to plants anyw

Tap water often contains high levels of minerals, and over time they precipitate in the dirt the plant grows in. Mostly calcium, which is useless to plants anyway. As they accumulate, the pH of the dirt increases, until the concentration is higher than that of the inside of the roots and osmosis stops working, and the plant cannot absorb nutrients anymore. Then usually, it starts showing signs of nutrient deficiency, so the first idea the gardener gets is too give them fertilizer, which increases the concentration even more and makes the problem worse. Rain water is very poor in minerals and acidic and doesn't cause this problem. It's also possible to just use rain water from time to time, to leach out the minerals in the dirt. Another solution I've heard when using tap water is to always pour it until it flows from below, and throw away the water in the saucer, it helps prevent the accumulation from water going in and never out and never having a chance to leach out minerals. Some plants (most common houseplants like this one) can tolerate elevated levels of minerals, so I don't think it's really a problem here.