Well that gives me a pretty simple way to go about doing it, but how do you measure the amount of water taken out for a water change? And then how would you figure out the amount of CaS04, KHC03, MgS04, and minerals to use in said amount of water.
Measure it however you want.
I would take a 5 gallon pail, fill it using a large measuring cup and mark the 5 gallon level when you reach it, now you have a near perfect 5 gallon reference mark, and fill to that level, dump, repeat until you reach whatever percentage of a water change you are after.
Mark the tank corner with tape / marker / whatever will stay put, and you never have to measure out anything again.
Get a brute trash can / heavy duty storage tote to use as a mixing container for your minerals.
Again, use the measuring cup / 5 gallon pail / tape / marker method to make a reference point in the mixing container (or see my example below for a way to do it by saving water, using your first water change's water as the measured amount to give you a reference point in your mixing container) Now you can add fresh water to your mixing container up to the reference point which will exactly = your water change amount removed from the tank.
Use the rotalabutterfly calculator to determine how much of each compound to add into your mixing container to yield your desired nutrient / kH level.
Once you do it once, it will only take 5 minutes. A small powerhead or pump is useful uin mixing up the dry ingredients into your mixing container full of water, and for transferring that water up into your tank.
It doesn't have to be fancy, keep the mixing container in a closet or tucked away some where, pull it out on water change day, fill it up with RO water or RO / Tap mix to your reference mark, add your ingredients, and use a spare heater if you want it to match your tank water temp more closely. Let it mix / heat up for an hour and it's ready.
Siphon out the tank to your reference mark, and then transfer the water from the mixing container into the tank.
FOR EXAMPLE:
75 gallon tank, with 65 gallons of actual water volume (or whatever your number works out to be) that you measured upon initial fill up using the measuring cup / 5 gallon pail / reference mark method, with 50% water changes:
65 / 2 = 32.5 gallons of water to be changed.
Siphon out 32.5 gallons using the 5 gallon pail. Remove exactly 5 gallons at a time until you reach 6 pails (30 gallons) then remove 2.5 gallons using the measuring cup (I recommend a large measuring cup, it should be accurate enough for our needs, and you're not sitting there removing 1 cup at a time).
Dump each bucket with exactly 5 gallons of tankwater into your mixing container, preferably near the drain. Once you've removed 6 x 5 gallons, and 1 x 2.5 gallons from the tank and added it to the mixing container, place a reference mark wherever it best fits and will stay and is visible to you when filling. That reference mark represents exactly 32.5 gallons. Dump the tank water down the drain.
Now the tank is exactly at 50% capacity, place your reference mark on the outside of the least visible back corner of the tank. That mark represents exactly 32.5 gallons removed.
Now you have reference marks on both the tank level @ 50% removed, and the mixing container at exactly 32.5 gallons.
You can then punch that into rotalabutterfly, or use this:
30ppm Ca = 0.488 grams CaSO4.2H2O (Gypsum) per 1 gallon
10ppm Mg = 0.384 grams MgSO4.7H2O (Epsom Salt) per 1 gallon
1.0 Degrees kH = 0.135 grams KHCO3 (Potassium Bicarbonate) per 1 gallon
Multiply these numbers by your amount of measured gallons upon initial fill, and then the amount of gallons per water change (In my example it would be 65 gallons and 32.5 gallons).
You want these pre-mixed into your new water prior to adding it to the tank, this will ensure no shock to livestock during the water change (mineral contents will match).
You can initially fill the tank to measure out how much actual water volume you have, and then add these into the tank. This will ensure accuracy and there is no livestock to worry about at that time.
