Are ppm and mg/L the same unit?

In an aqueous solution, yes: 1 mg/L = 1 ppm (one part per million). Since 1 L of water weighs about 1 kg, concentration × volume gives milligrams directly — 147 L at 0.2 mg/L is 29.4 mg of the pure substance.

How much fertiliser per day?

For a 900×450×450 tank (147 L net) with a 10 g/L stock and a 0.2 mg/L target: 2.9 mL per dose, carrying 29 mg of the pure substance. Topping up three times a week is about 8.8 mL, so a 500 mL bottle lasts roughly 170 doses. A high-tech planted tank usually targets 0.5–1 mg/L; scale the volume in proportion.

Why does dechlorinator use the whole system volume?

Because chlorinated water circulates through everything — filter chambers, plumbing and the voids in the substrate — not just the part the fish swim in. The tool estimates the whole system as the inner cavity × 1.15, which is 187 L in the default case against 147 L of swimmable water. Skip that extra third and chlorine survives the water change.

How does a dip concentration relate to the tank?

It does not, directly. A dip uses a separate container with little water and a much higher concentration: 20 L at 2 mg/L needs 40 mg of the pure substance. It is listed separately precisely so that nobody doses a dip concentration into the display tank.

How do I find the stock concentration?

Read the label: a serious fertiliser states grams of N-P-K per litre, or an overall concentration. If it only says "add 5 mL per 100 L", use the bottle-recommendation mode — the tool back-calculates the effective concentration from the stock figure you enter.

What happens if I overdose?

Algae benefit first, and then the plants suffer reverse osmosis at the roots. The signals are a sudden bloom of green spot algae on the glass and blackened edges on older leaves. Change 30–50% of the water and restart at the low end of the range.

A-01

Dosing & Medication

Bottles usually say "add so many mL per 100 L" while test kits read mg/L (ppm). The gap between the two is the stock concentration — which is why it is the first input here: at 10 g/L, 1 mL carries 10 mg of the pure substance. For the same target, doubling the stock concentration halves the volume.

Fertiliser volume follows from target concentration × net water: a 900×450×450 tank holds 147 L of net water, so 0.2 mg/L needs just 2.9 mL of a 10 g/L stock solution.

Units

Input parameters

Tank dimensions

Tank length Win

Tank depth Din

Tank height Hin

Hardscape share%

Stock solution and target

Stock concentrationg/L

How you measure

Target concentrationmg/L

Dip / quarantine container

A separate container with its own volume and concentration, unrelated to the display tank. Leave 0 to skip.

Dip volumeUS gal

Dip target concentrationmg/L

Result

Swimmable water (hardscape deducted): 38.8 US gal
Whole system volume (filter and plumbing included): 49.5 US gal
Stock solution per doseEquivalent to 0.029 g of the pure substance: 0.1 US fl oz
Effective concentration: 0.20 mg/L
Weekly use (3 top-ups): 0.3 US fl oz
A 500 mL bottle lasts: about 170 doses
Dechlorinator (full water change): 0.3 US fl oz

Notes

1 mg/L = 1 ppm, and 1 L of water weighs about 1 kg, so concentration × volume gives milligrams directly
Stock concentration is **grams of pure substance per litre of solution**: at 10 g/L, 1 mL carries 10 mg
Dechlorinator is dosed against the **whole system volume** (filter and plumbing included, estimated as inner volume × 1.15), not the swimmable water
A dip container is far smaller than the tank but far more concentrated: mix it separately and never dose it into the display tank
Temperature affects both the medication and dissolved oxygen; keep it steady at °C and add an air stone during treatment

The calculator above follows the unit system selected at the top of this page; the explanatory text below stays metric (mm / L / kg / ℃).

Need a full design?

Want to turn the dimensions into a 3D model and a complete bill of materials: Enter length, width and height in the aquarium designer and it generates the 3D model, dimensions, glass cut list, silicone usage and equipment selection, all exportable as CSV and PNG.

How to use it

Step 1Measure the tankNet water comes from the inner cavity less the hardscape share; the whole system volume (filter and plumbing included) is estimated as the inner cavity × 1.15.
Step 2Enter the stock concentrationLook for grams of pure substance per litre on the label; common fertilisers run 5–20 g/L. Get this wrong and everything downstream is wrong.
Step 3Choose how you measure and enter the targetIf you have a test kit reading, pick "target concentration" (mg/L). If you only want to follow the bottle, pick "bottle recommendation" (mL/100L).
Step 4Leave the dip at 0 if you are not treatingThe dip volume is a separate container unrelated to the display tank; 0 skips that calculation.

Frequently asked questions

Are ppm and mg/L the same unit?: In an aqueous solution, yes: 1 mg/L = 1 ppm (one part per million). Since 1 L of water weighs about 1 kg, concentration × volume gives milligrams directly — 147 L at 0.2 mg/L is 29.4 mg of the pure substance.
How much fertiliser per day?: For a 900×450×450 tank (147 L net) with a 10 g/L stock and a 0.2 mg/L target: 2.9 mL per dose, carrying 29 mg of the pure substance. Topping up three times a week is about 8.8 mL, so a 500 mL bottle lasts roughly 170 doses. A high-tech planted tank usually targets 0.5–1 mg/L; scale the volume in proportion.
Why does dechlorinator use the whole system volume?: Because chlorinated water circulates through everything — filter chambers, plumbing and the voids in the substrate — not just the part the fish swim in. The tool estimates the whole system as the inner cavity × 1.15, which is 187 L in the default case against 147 L of swimmable water. Skip that extra third and chlorine survives the water change.
How does a dip concentration relate to the tank?: It does not, directly. A dip uses a separate container with little water and a much higher concentration: 20 L at 2 mg/L needs 40 mg of the pure substance. It is listed separately precisely so that nobody doses a dip concentration into the display tank.
How do I find the stock concentration?: Read the label: a serious fertiliser states grams of N-P-K per litre, or an overall concentration. If it only says "add 5 mL per 100 L", use the bottle-recommendation mode — the tool back-calculates the effective concentration from the stock figure you enter.
What happens if I overdose?: Algae benefit first, and then the plants suffer reverse osmosis at the roots. The signals are a sudden bloom of green spot algae on the glass and blackened edges on older leaves. Change 30–50% of the water and restart at the low end of the range.