The main concern with tap water would be the additives and contaminants that can directly affect your plant’s nutrients. These contaminants come in the form of calcium, magnesium, rust, chlorine, chloramine, and more.
Though the water coming out of the tap is considered "safe," it isn't pure at all. By using a tool called a TDS Meter (total dissolved solids), you would be able to see a reading that can range from 200-300(ppm) parts per million. This number can vary based on where you live. But pure water has a reading of 0ppm.
Hydroponics nutrients use 0ppm as a base for their dosage instructions. The instructions on the side of the bottle or packing assume that you are using pure water. When tap water with hundreds of ppm is used, then overfeeding your plants may occur.
It is suggested that when mixing nutrients, you would need to start off with the purest form of water possible, this is achieved by using a reverse osmosis system (RO). This type of water filtration has multiple stages, and each one has a specific function in the filtration process. The first stage will remove any large particles like sediment, silt, dust, and rust. The second stage is a 5-micron coconut shell carbon block. This is in place to reduce chlorine, improve taste, odors, and remove chemicals that are commonly found in municipal water. The third stage is a membrane that rejects contaminants such as bacteria, viruses, fungi, lead, radium, arsenic, and heavy metals. The final stage is a coconut shell carbon block that removes any residual, unnatural water tastes that were not removed from the previous states, which in turn adds a bit of sweetness from the coconut shell carbon.
One of the problems with hard water would be the high mineral count. Though plants need calcium and magnesium, they only need a small amount. Straight tap water could pose a risk of mineral oversaturation. Another scenario that can occur when using tap water is the mineral molecules in the water are too large for plants to absorb. Furthermore, hard water can become problematic with hydroponic equipment such as pumps, filters, and fittings by creating limescale buildup.
Having an RO system is a great way to start with a good foundation for a hydroponics environment. This allows the grower to customize their water profile to fit the needs of their plants. As water evaporates, having RO water readily available to replenish the system will lessen the impact of pH level fluctuations and nutrition deficits.
So to answer the question “is tap water safe for hydroponics?” The short answer is, yes. Municipalities have to meet a very strict standard in water quality in order for it to be potable. That means the water that comes through the tap has to be safe enough to drink, bathe, and cook with, and ultimately, it’s safe enough for plants. However, RO water can have a positive impact on the overall quality and quantity of the plans you are growing in your hydroponics system.