Hello: This is the sort of unit you’re looking for. http://www.chromalox.com/productcatalog/Packaged+Systems+and+Skid+Systems/Large+Tank+Heaters/Open+Coil+Element+%28OCE%29+Pipe+Insert+Large+Tank+Heaters/product-details.aspx?p=289 I would call Chromalox (1-800-443-2640) and talk to an engineer. You likely need length, diameter, voltage and wattage for them to be able to help you. These used to be pretty easy to find…. they simply get inserted into pipes running horizontally in the tank, so water doesn’t need to be turned off to change them. Best of luck!
ps: This is what you’re looking for if the link above doesn’t work as it should.
Large Tank Heaters [list]