Crack in fresh water tank

If you can't easily remove the tank but can reach the cracked fitting I would try an adhesive like Shoe Goo or Goop sold in Walmart and reinforce with gauze sheets. Drain and sand the area well clean with acetone. You want the acetone to attack the surface for a good bond. I assume the drain is under the tank so the excess solvent won't puddle. Apply about a .030 layer of adhesive then wet out the gauze sht and apply the wetted material to the .030 layer smoothing this with your fingers removing any entrapped air. If the crack is larger than .030 apply several layers. I have used this several times to fix cracks in plastic tanks.