To fix a hole in your water tank, try T-Rex 48mm x 1.5m Strong Waterproofing Tape as it is a butyl adhesive tape that sticks to wet surfaces and works underwater, making it ideal for stopping active leaks.
Alternatively, you might like to try PC Products 57g PC Marine Epoxy Putty. Clean the area thoroughly and sand around the hole with 80 grit sandpaper. You can then mix up a small amount of the putty and push some into the hole and smear the rest over the area.
