Coolant sin't a consumable, so if you're low, then it's leaking. Coolant should be replaced about every 2 years. If longer alkali can build up and the pH becomes acidic chewing away at the water jackets. If you have a hygrometer you can check with a drop of your coolant. You can also take a DMM and check to see if voltage is being produced.

0.0- 0.2V good

around 0.5 V think about replacing


0.7V+ change it now!!!