Got a stock or concentrate tank that's only part full? Tell us how full it is and what a full tank of your recipe needs, and we'll work out exactly how much of each ingredient to add to fill it back up. Handy when you've moved to a new life stage but still have solution left from the old dose.
A 50 gallon tank with 20 gallons left is 40 percent full. Filling it with plain water leaves the whole tank at 40 percent of recipe strength. Each fill with water alone lowers the strength again.
The solution still in the tank carries the same share of each ingredient as it does of the tank volume.
Still in tank = full-tank amount × (amount left ÷ tank size)
Add to top off = target amount − still in tank
| 50 gal tank, 20 gal left | Full tank | Still in tank | Add |
|---|---|---|---|
| Calcium nitrate | 3,000 g | 1,200 g | 1,800 g |
| Magnesium sulfate | 1,500 g | 600 g | 900 g |
| Water | 50 gal | 20 gal | 30 gal |
With the same recipe, the amount to add is the full-tank amount times the fraction already used, 60 percent in this example.
A change in growth stage often calls for a new recipe while old solution is still in the tank. With the recipe switch turned on, the tool subtracts what the old solution holds from the new recipe's full-tank amounts. A negative result means the tank already holds more of that ingredient than the new recipe calls for. Nothing can be taken back out, so the options are to accept the higher level or drain more of the tank and run the numbers again.
The water figure is the empty space in the tank. Ingredients measured in liquid units (mL, L, fl oz, cups, tsp, Tbsp) take up some of that space, and their volume is subtracted. Dry ingredients measured by weight also take up volume, but how much depends on the salt, so the tool leaves it out. With dry ingredients, filling to the full line gives the right volume.
The math is the same for a concentrate tank feeding an injector and for a reservoir at feed strength. For a concentrate, the injector ratio does not change. At stock strength, calcium nitrate goes in a separate tank from sulfates and phosphates, because together they form insoluble precipitates.
After the top-off is mixed, the EC of the tank, or of the diluted feed from an injector, should match the reading from a fresh full batch of the same recipe.