Salt is a natural bacteriacide and will also thicken a fishes slime coat which aids resistence to disease (white spot, parasites, etc) or chemical imbalences (ammonia burn, pH shock etc). Some people add it regularly as a back ground preventative dose and some people use it purely as medication as required. I buy swim pool salt in 25 Kg bags and have enough experience to guestimate dosages by the handful. Toss it in and let it dissolve. You could get a measure scoop or cup if you want to be more cautious. Rather than work out how much salt to add to 100L I would work out how many litres a level cup does. Once you work this out, its easier and faster to work out how many cups for each size tank.
