Vinegar and water, soak the powerhead etc in it, I do it overnight, and agitate the article to dislodge the calcium buildup, then soak in plain water overnight, rinse well. If it is in a tank that is dry.....not in an established tank, use a razor blade with vinegar.....only if the tank is glass....never on an acrylic tank. What are you trying to rid the buildup on or in? Meaning the back glass? Unfortunately if the tank has a lot of calcium deposits, from neglect of cleaning off the sides and front glass, then this is where your hard work is going to have to come into play.
Vinnegar and water is your best bet, maybe just soak the tank overnight in it, then start scrubing, just be sure to rinse really well. I mean really well..........