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ppm means “part per million,” so if the desired concentration in the tank is 10 ppm that means that there are 10 parts of the chemical to 1 ,000,000 parts of water.

Dry compound is highly concentrated, it is essentially 99 % chemical to 1% water that has condensed out of the air.

If you take 1 gram of dry chemical and add it to 1 liter of water then add the 1 gram to a swimming pool of 100,000 liters your concentration of chemical is much higher in the 1 liter than in the pool since the 1 gram of chemical is much less spread out than it is in the pool.

Now if you take 1 mL from the 1 liter (1,000 mL) that was just mixed with 1 gram of chemical you are basically taking 1/1000th of a gram out. Compare this with taking 1 mL from the pool, which works out to be 1/100,000,000th of a gram of chemical in the 1 mL.

Adding liquid from a pre-mixed bottle of water is much easier and more precise when dosing your tank, because if you only need 1/1000th of a gram of chemical in the tank water how are you going to measure out 1/1000th of a gram of dry chemical without a really expensive scientific scale? It is far easier to get the tiny fraction of a gram of chemical that is required via diluting it down than add dry chemical directly, although if you had an accurate enough scale and good enough tweezers to pick up individual grains of chemical then it would work out to be the exact same thing in both cases.

Adding teaspoons of dry chemical to the tank isn’t usually a good idea, because it usually results in massively overdosing the tank with chemical (to the point of toxicity).
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