Water is an essential ingredient when growing marijuana indoors. The quality and purity of your water source will definitely affect the outcome of your harvest. While tap or municipal water works well for the most part, if you are a serious cannabis grower, you want full control over as many variables as possible and must consider a reverse osmosis water filtration system.Why not tap, municipal or well water? Water is water, right? Partly right in that H2O is H2O, but the things floating around in it vary widely from location to location. Chlorine and/or chloramine used as disinfectants are found in large quantities in all drinking water. Other common chemical contaminants include salts, nitrites, herbicides, pesticides, arsenic and BTE, a gasoline additive. Some areas of the country (USA) have extremely high concentrates of minerals, notably calcium carbonate and magnesium carbonate. This is known as ‘hard’ water. Not only can hard water affect your plants, it can leave mineral deposits in your pumps, tubes and easily clog up spray heads. If you are growing weed with aeroponics, RO is a must!pH is a measure of the acidity or basicity of an aqueous solution. This is very important to know as marijuana has an optimal pH range where it will thrive. By controlling the pH of the source, it is much easier to adjust (if necessary) and to get consistent growth. Pure water will vary from 6 to 7 on the pH scale depending on dissolved air and temperature.The other main reason to control your water supply is to limit any water-borne pathogens such as bacteria, protozoans and viruses. Read more about pH here.Editors' Pick - LED Grow LightsKIND LED Grow Lights - 12 Band Spectrum Most hydroponic nutrient manufacturers supply a feeding chart. This assumes as starting point of zero PPM (parts per million of dissolved solids) yet municipal water mad have a starting PPM of 300. Without knowing the exact makeup of the dissolved solids it can be quite easy to over or underfeed your plants. Review of PPM and pH meters here.What is a reverse osmosis water filter? It is the process of forcing unfiltered water though an extremely fine membrane under high pressure leaving the contaminants behind. Most systems have three separate filters: sediment pre-filter to remove larger particulate matter; an activated carbon pre-filter to remove chlorine and other chemicals and the osmotic membrane which removes virtually all other contaminants. The pre-filters not only cleanse the water but protect the membrane from damage. Some units use an additional activated carbon post-filter.Vending machines For the small-scale cannabis grower, one and five gallon jug vending machines are an easy solution as they have appeared outside many grocery and convenience stores in the last few years. Those with a larger gardens may want to consider an in-house filtration system.Reverse osmosis water filtration systems Commercial stems may range from a few hundred to many thousands of dollars depending on features, quality, and flow rate. Another consideration is the ratio of waste water to purified water. Also factor in the cost of replacement filters.If you want to take growing marijuana indoors to the next level, then consider using an RO water filtration system.