How can I repair a cracked water tank?
Try using a windshield repair kit like the one shown here:
https://www.thoughtco.com/repair-your-chipped-windshield-281417

The basic steps are:


Clean the crack and area around it
Drill a small hole in front of each crack and chip at the hole to create a 'bulls eye', this helps stop the cracks from spreading
Align a resin applicator tool with the crack
Load a windshield resin applicator with resin and begin spreading the resin into the crack
Apply a finishing film to smooth the resin.
Let dry
Scrape away excess resin with a razor.