The concrete is either a minimum of 4,000 or 5,000 PSI concrete. A 1,200gallon tank can weight as much as 8,000 pounds, so these are not items a homeowner can install on his own. Tank dimensions vary somewhat by manufacturer, but a 1,000 gallon septic tank, usually is 8 feet, 6 inches long by 5 feet, 5 inches tall by 4 feet, 10 inches wide.