It's just the standard elemental wheels: Fire > Water > Thunder > Earth > Wind > Ice > Fire and then Light <-> Dark. So Fire leaks, you'd use Ice, Thunder leaks you'd use Water, and Dark leaks you'd use Light.
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Please read what I said earlier. "To deal with leaks, you have to use the element weak to the leak's element."
So let's go back to your question. What is weak to water? Fire is weak to water, so you'd use fire to patch a water leak.
The list I showed above was simply an explanation of how the elemental wheels worked, if anyone in the game at this point still doesn't know. Fire loses to water, water loses to thunder, thunder loses to earth, earth loses to wind, wind loses to ice, ice loses to fire.