Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Copper dose to reach a therapeutic level in a quarantine tank, from zero or from a measured reading.
Copper has a narrow window between working and lethal, so test daily with a good kit. Hanna and Salifert copper kits both work. API does not read the range you need. Never use copper in a display tank, because rock and sand absorb it and release it for years.
mL = (2.00 − 0.00 ppm) × 75.7 L ÷ 6.4 = 23.66 mL
Copper products are dosed to a target concentration in the water, so the arithmetic is the same as any other addition: concentration gap times volume divided by product strength.
The strengths differ a lot between products and they are not interchangeable. Seachem Cupramine reaches 0.25 ppm from 1 mL per 40 L, which works out to 10 ppm per mL per litre. Copper Power is 6.4 and Coppersafe 5.6 on the same basis.
Target levels differ too, and mixing them up is dangerous in both directions. Cupramine is therapeutic at 0.5 ppm, while the chelated products need 1.5 to 2.0 ppm. Dosing a chelated product to 0.5 does nothing, and dosing Cupramine to 2.0 kills the fish.
Seachem specify Cupramine in two half doses 48 hours apart rather than one. The fish tolerate the approach to therapeutic level better that way.
Test daily. Copper binds to any organic surface it can find, so the level drops on its own, and a tank that tested right yesterday can be below therapeutic today. Use a Hanna or Salifert copper kit, since the general purpose kits do not resolve this range.
Every water change removes copper in proportion to the water removed. Replace it at the target concentration for the volume changed, which is what the replacement figure above gives.
Copper is lethal to all invertebrates and is absorbed permanently by rock, sand, and silicone. Treat in a bare bottom quarantine tank with PVC pipe for cover, and never put copper in a display. A tank that has held copper is not reliably safe for inverts afterwards.
Run the full course, usually 14 to 30 days at therapeutic level depending on the parasite and the product. Stopping early selects for the survivors and wastes the fish's time in quarantine.
Sources: Seachem Cupramine dosing · Fritz Mardel Coppersafe · DailyMed, Coppersafe composition