figured I'd add this, after you got a couple good replies ( agree with vinegar / vinegar salt... throw in a razor if it's a glass tank ( but not for acrylic use ))
when I was a kid, I used to have a 55 gallon with 3 red breasted piranhas, that I used to feed chunks of raw chicken liver ( we raised chickens, & no one seemed to want to eat the livers ) of course after feeding them, the water would get cloudy for a half day or so... one day my littlest brother walked in, & saw the cloudy water, & thought he would help me out by running to the kitchen & getting the dish soap... as you could guess I walked in an hour or so later & the air stones had formed a 2 foot high drift of bubbles flowing on to the floor... of course my 3 - 8 inch long piranhas were dead... no idea how much soap he squirted into the tank...
BTW, I have used soap before, but it requires a lot of rinsing, & it does not much in the way of helping remove hard water build up... the 1st 2 suggestions are likely the best you'll get, IMO...