What can I put in the hot water heater to prevent mineral deposits?

You cannot. You need to install filtration system that will service whole house and / or water conditioning system.

what can I put in the hot water heater to prevent mineral deposits?
Milo's correct! However I have a much less costly solution.
Flush the tank on a regular maintenance schedule. Let me show you how.
For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule. Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes. Do this monthly to keep it clear. . Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps and thank you for rating my answer, Tom