A water tank is a container for storing water, for many applications, drinking water, irrigation, fire suppression, farming, both for plants and livestock, chemical manufacturing, food preparation as well as many other uses. Water tank parameters include the general design of the tank, and choice of construction materials, linings. Various materials are used for making a water tank: plastics (polyethylene, polypropylene), fiberglass, concrete, stone, steel (welded or bolted,^{[citation needed]} carbon, or stainless). Earthen pots, such as matki used in South Asia, can also be used for water storage. Water tanks are an efficient way to help developing countries to store clean water.^[1]

Throughout history, wood, ceramic and stone tanks have been used as water tanks. These containers were all naturally occurring and some man made and a few of these tanks are still in service. The Indus Valley civilization (3000–1500 BC) made use of granaries and water tanks. Medieval castles needed water tanks for the defenders to withstand a siege. A wooden water tank found at the Año Nuevo State Reserve (California) was restored to functionality after being found completely overgrown with ivy. It had been built in 1884.^[2]

Chemical contact tank of FDA and NSF polyethylene construction, allows for retention time for chemical treatment chemicals to "contact" (chemically treat) with product water.

Ground water tank, made of lined carbon steel, may receive water from a water well or from surface water, allowing a large volume of water to be placed in inventory and used during peak demand cycles.

An elevated water tank, also known as a water tower, will create a pressure at the ground-level outlet of 1 kPa per 10.2 centimetres (4.0 in) or 1 psi per 2.31 feet (0.70 m) of elevation. Thus a tank elevated to 20 metres creates about 200 kPa and a tank elevated to 70 feet creates about 30 psi of discharge pressure, sufficient for most domestic and industrial requirements.

Vertical cylindrical dome top tanks may hold from 200 litres or fifty gallons to several million gallons. Horizontal cylindrical tanks are typically used for transport because their low-profile creates a low center of gravity helping to maintain equilibrium for the transport vehicle, trailer or truck.

A Hydro-pneumatic tank is typically a horizontal pressurized storage tank. Pressurizing this reservoir of water creates a surge free delivery of stored water into the distribution system.

Glass-reinforced plastic (GRP) tanks/vessels are used to store liquids underground.^{[citation needed]}

By design a water tank or container should do no harm to the water. Water is susceptible to a number of ambient negative influences, including bacteria, viruses, algae, changes in pH, accumulation of minerals, and accumulated gas. The contamination can come from a variety of origins including piping, tank construction materials, animal and bird feces, mineral and gas intrusion. A correctly designed water tank works to address and mitigate these negative effects. It is desirable that water tanks be cleaned annually to reduce delivery of algae, bacteria and viruses to people or animals.^{[citation needed]}

A safety based news article linked copper poisoning as originating from a plastic tank. The article indicated that rainwater was collected and stored in a plastic tank and that the tank did nothing to mitigate the low pH. The water was then brought into homes with copper piping, the copper was released by the high acid rainwater and caused poisoning in humans.^{[citation needed]} Since the plastic tank is an inert container, it has no effect on the incoming water. Good practice would be to analyze any water source periodically and treat accordingly, in this case, the collected acid rain should be analyzed, and pH adjusted before being brought into a domestic water supply system.

The release of copper due to acidic water may be monitored by a variety of technology, beginning with pH strips and going to more sophisticated pH monitors, indicate pH which when acidic or caustic, some with output communication capabilities. Most of the algae growth occurs at an optimum pH,  between 8.2 - 8.7. pH level that is neutral or lower can help to reduce the growth of algae. Potential algaecide, shock product will help to clean swimming pools or water tanks from algae. In this process no need to use vacuum cleaner to remove algae.^{[citation needed]} There is no causative link between the plastic tank and copper poisoning, a solution to the problem is to monitor stored rainwater with pH indicators and add appropriate treatment materials.

Recent advancements in water tank inspection and maintenance have significantly enhanced system safety and longevity. Key among these technologies are remotely operated vehicles (ROVs) and thermal imaging, which have become instrumental in early detection of potential issues.

ROVs offer a non-intrusive means to inspect water tanks, allowing for detailed examination without direct human entry, thereby increasing operational safety and efficiency.^[3] Thermal imaging, on the other hand, is particularly effective in low-visibility and harsh environments, as it facilitates the identification of temperature anomalies that may indicate leaks, weaknesses, or other faults within the tank structure. This application of thermal imaging in structural health monitoring has been substantiated in recent studies,.^[4][5] Together, these technologies enable comprehensive diagnostics that surpass traditional inspection methods, ensuring water tanks meet the highest standards of reliability and regulatory compliance.

• Drinking Water Contaminants - Information from the United States Environmental Protection Agency
• American Water Works Association - Official website
• Technical Reports: More Than Just Compliance
• Boosting Efficiency in Tanks with Thermal Imaging
• Infrared Thermography Measurement for Vibration-Based Structural Health Monitoring in Low-Visibility Harsh Environments