The tank shape determines how the material will resist the applied force and thus how easy it will be to resist a given load. This is something that you should consider carefully if you are making or modifying a tank.
There are plenty of choices for water tank materials, each with advantages and disadvantages. Common materials include galvanized steel, various plastics, fiberglass, concrete, ferrocement, brick, and rock and mortar. However, a few circumstances are unequivocally hazardous and to be avoided:
PVC exposed to sunlight—PVC breaks down in sunlight, reacting to form carcinogens, which leach into the water. It is a plumbing code violation to have potable water in un-shaded PVC for this reason. You can see physical evidence of the change on the outside of the pipe; it darkens, becomes chalky and brittle. The reaction progresses from the outside in. To the extent PVC should be used at all, it should be buried or indoors. If you have PVC that has already degraded, you should replace it.
Pre-1997 PVC—which was made with more toxic plasticizers.
Flexible PVC water bed bladders or trash cans—which contain a high level of toxic plasticizers.
Pre-1980 tank coatings including coal tar and lead-based paint—These were great for corrosion resistance but oops!—they poison the water.
Lead pipe and pre-1987 lead-soldered copper pipe—Solders and flux currently contain less than 2% lead. Before 1987 they typically were half lead. Lead pipe can be recognized by its softness.
Western red cedar—The same stuff that smells good and keeps it from rotting is toxic when ingested.
Fly ash in concrete—especially when exposed to acidic water.
Often the worst hazards are not the base material, but solvents, additives, mold-release agents, fungicides, etc.
Many, if not most, water tanks are installed with little or no regulator involvement, but rules and enforcement can vary quite a bit depending on where you are. You’ll need to inquire locally to find out what you’ll be subject to.
Water storage may be subject to zoning, building department, fire department, and health department rules. If you have a homeowners’ association, it may regulate water tanks, perhaps just because they are a structure. Your insurance company may have rules or incentives relating to water storage as a fire safety resource, a flooding hazard, or simply as another asset to insure.
Some of the rules you may encounter will be consistent with your own interests, and some will run counter to them. You may run into rules concerning:
You may or may not be allowed to build a water tank within the building setbacks from your property lines—the zoning department can tell you.
In some neighborhoods, rules may prohibit above ground tanks. A beautiful ferrocement tank with an attractive shape and color, or one that looks just like a boulder, may be able to overcome anti-tank prejudice.