Calculate the correct medication dose for your aquarium's actual water volume. Accounts for substrate and décor displacement so you don't under- or overdose.
Dosing medication to the wrong water volume is the single most common cause of treatment failure and accidental fish kills. The label assumes a clean tank with no hardscape, no substrate displacement, and a known volume, but real tanks have rock, wood, equipment, and substrate that can occupy 10-25% of the listed gallons. This calculator takes the label dose, label volume, your tank's gross volume, and a decoration-displacement percentage and returns the correct dose to add, expressed in the same units as the label.
The calculator first computes actual water volume by subtracting decoration and substrate displacement from gross tank volume. It then scales the label dose linearly: actual_dose = label_dose * (actual_volume / label_volume). Units are preserved so milliliter labels yield milliliter doses and drops yield drops. A reminder is shown that a US gallon is 3.785 L (not 3.79 L sloppy rounding) and that some medications, particularly copper-based reef treatments, require precise dose against actual water volume to avoid invertebrate kills.
Enter your tank's total volume, the percentage of that volume occupied by substrate, rocks, wood, and equipment, the dose listed on the medication label, and the volume that dose is designed to treat.
Actual water volume is always less than tank capacity. A 100 L tank with a 4 cm gravel bed and rock work may hold only 75–80 L of actual water. Dosing based on nominal tank size leads to underdosing.
Always remove activated carbon before medicating — carbon absorbs most fish medications and renders treatment ineffective. Re-add carbon after treatment completes to remove residual medication.
Gravel, rocks, wood, and equipment displace water, reducing actual water volume. A 100-liter tank with 20% décor holds only 80 liters of water. Dosing based on full tank volume means underdosing, which can fail to treat the disease.
Light décor (small plants, minimal gravel): 10–15%. Moderate décor (standard gravel bed + rocks): 15–25%. Heavy décor (thick substrate, large rocks, driftwood): 25–40%. When in doubt, measure by displacement — fill a bucket with décor and measure water needed to cover it.
Yes — medication labels recommend dosing per volume of water, not tank size. Using actual water volume is the correct approach. Overdosing (dosing by nominal tank size when real volume is much less) is a common cause of fish deaths during treatment.
Yes. Activated carbon absorbs most medications, rendering treatment ineffective. Remove carbon media before adding medication. You can add carbon back after treatment is complete to remove residual medication from the water.
Generally no — combining medications increases the risk of toxicity. Treat one condition at a time. If multiple diseases are present, choose the most critical one first. Always follow the manufacturer's instructions.