Use a strong solution of vinegar. If you can find pickling vinegar that would be best because it is more acidic. Other then that cheap no-frills white vinegar. If a reverse osmosis water filter is affordable for you then it may be worth it to invest in one. You can use it for water changes as well as drinking water. You will have to get a trace mineral additive to put in the water for the aquarium but it will solve the calcium build up problem and reduce maintainance. I purchased a 6 stage RO filter on eBay with the tap for the faucet and 4 gallon storage tank for about $120. The ammount of filtered water it produces it paid for itself 10 fold in the first year if I had bought "purified" drinking water or spring water.

CLR for cleaning it. Just be sure to rinse it very very well before putting it back on top of the tank.

I have very hard water and have to remove all my faucets about once a month to let them soak.