I was Hoping to get a comment on which calculator to use. I don’t think I may have explained myself correctly.
@vetteguy53081 is correct - these calculators are just mathematic models based on the bottle dosing schedule. The key variable is tank volume, and many people get that wrong. Then, the calculator runs on "garbage in, garbage out". The best thing to do is use one of the calculators to get you close (they both do that) and then do your fine tuning of the dose with a Hanna Checker. What I like to do is calculate the amount of medication, than add 50%, let it mix well and test to see where I'm at, and redose accordingly.
Here is a writeup I did on dosing and tank volumes:
Medication dose calculation
Dosing medications in the United States is confounded by the fact that we tend to measure medications using metric units but tank volume is usually measured in gallons. The basic equation for calculating these medication dosages is as follows: Parts per million projected dose times net gallons of water divided by 266 equals grams or milliliters of medication to be added.
X ppm x Y gallons / 266 = Z grams of medication
(Since a milliliter of most liquids weighs one gram, volume can often be used interchangeably.)
Sample: You need to dose a tank that holds 25 gallons of water with formalin at 75 parts per million. How much do you add?
25 x 75 = 1875
1875 ÷ 266 = 7.05 ml of formalin.
Rounding this value off, you should add 7.1 milliliters of formalin to the tank to achieve the desired dose of formalin.
Tank volume calculations
For a rectangular aquarium: Length x width x height (in inches) divided by 231 will give you the base gallon capacity of the tank.
To refine this base capacity:
Measure the inside of the tank from the average surface of the gravel to the waterline for the height measurement described above. Then calculate the volume of your gravel bed. To do this, multiply the depth of the gravel by the width and length of the aquarium. Divide this by 231. Finally, divide this result by 3. This will give an approximate volume of the water contained between the grains of gravel in your filter bed. Add this value to your base gallon capacity.
Next, you need to subtract water displaced by any tank decorations, live rock, etc. There is no easy way to do this. For a standard "fish aquarium" decorated with a few pieces of coral, the amount to be subtracted is typically around 15% of the total calculated volume.
Finally, use the same method to calculate the volume of water held in the sump, if applicable, and add it to the running total.
Of course, the best way to determine the volume of a given tank is to determine the number of gallons it takes to fill it for the first time. Small aquariums can be filled with a vessel of known volume. Larger public aquarium exhibits typically need to be filled using a metered flow.
