Quick answer: This aquarium fertilizer calculator, also known as an aquarium nutrient calculator, builds a full weekly EI, medium or low tech dosing plan for your tank, and it also tells you exactly how many grams of dry fertilizer to add for a target ppm, how many ppm a dose you already added gives you, and how strong a homemade stock solution is per ml. Enter your real water volume, pick the salt (KNO3, KH2PO4, K2SO4, Epsom salt, CSM+B or chelated iron), and it shows every nutrient that dose adds, not just the one you aimed for.
Weekly targets in ppm. They fill in for the style you pick, and you can change any of them.
Use your real water volume, not the tank's label size. Gravel, rocks and wood usually take up 10 to 20 percent of the space. Weigh dry powders with a 0.01 g jewelry scale for small tanks. ppm here means mg per liter.
How to use the fertilizer calculator
Full dosing plan. Start here if you’re new to dry fertilizers. Enter your water volume, pick your dosing style (high tech EI, medium, low tech or daily) and your water hardness. The calculator builds a complete weekly plan with grams of KNO3, KH2PO4, K2SO4, Epsom salt and CSM+B per dose and per week. It also works out how much potassium the nitrate and phosphate salts already add, so you only top up the rest with potassium sulfate. Every weekly target can be edited if you want to dose leaner or richer. Pick a PPS Pro style for small daily doses with little or no water changes.
Worst case build up. Under the plan you’ll see how high nitrate, phosphate, potassium and iron could climb with your weekly water change if plants used nothing at all. It’s the quickest way to see why EI needs a 50% weekly change, and why lean daily dosing suits people who change less water. If you run CO2, our aquarium CO2 calculator helps you match gas to nutrients.
Target ppm to grams. Use this when you know the dose you want. Pick the fertilizer, choose the nutrient you’re aiming for, and type the ppm. Example: to raise nitrate by 5 ppm in a 20 gallon tank, you need about 0.62 g of potassium nitrate.
Grams to ppm. Use this to check a dose you already use, or the amount printed on a recipe. Type the grams and it shows the ppm of each nutrient that salt adds.
Stock solution. Most people find it easier to dissolve powder in a bottle of water and dose with a syringe. Enter how many grams you dissolved, the bottle size in ml, and your dose in ml. The calculator shows the ppm per dose, how many ml equal 1 ppm, and how many doses the bottle holds.
One tip that saves a lot of confusion: use your real water volume. A 20 gallon tank with sand, rocks and driftwood often holds closer to 16 or 17 gallons of water. Dosing for the label size slightly overdoses, which is usually harmless, but it adds up with phosphate and iron. Our aquarium volume calculator works out your real water volume and sends it straight back here.
Fertilizers in this calculator
| Fertilizer | What it supplies | Main use |
|---|---|---|
| Potassium nitrate (KNO3) | About 61% nitrate and 39% potassium | Main nitrogen source in EI and PPS dosing |
| Monopotassium phosphate (KH2PO4) | About 70% phosphate and 29% potassium | Phosphate for healthy roots and green spot algae control |
| Potassium sulfate (K2SO4) | About 45% potassium and 18% sulfur | Extra potassium without adding nitrate |
| Epsom salt (MgSO4 7H2O) | About 10% magnesium and 13% sulfur | Magnesium for soft water or RO water |
| Gypsum (CaSO4 2H2O) | About 23% calcium and 19% sulfur | Calcium for very soft water |
| Plantex CSM+B | 6.53% iron plus manganese, boron, zinc, copper and molybdenum | All-in-one trace mix |
| Iron DTPA 11%, EDTA 13% or gluconate | Iron only | Extra iron for red plants |
Here’s the honest part most calculators skip. Every salt brings a partner nutrient along. Dose KNO3 for nitrate and you’re also adding potassium. Dose KH2PO4 for phosphate and you’re adding more potassium. That’s why the results table lists everything a dose adds, so you can see your full potassium number before you reach for potassium sulfate.
Typical dosing targets
These are common starting points, not strict rules. Watch your plants and adjust.
| Nutrient | High tech EI, per dose (3 times a week) | Low tech, per week |
|---|---|---|
| Nitrate (NO3) | 5 to 7.5 ppm | 2 to 5 ppm |
| Phosphate (PO4) | 1 to 1.3 ppm | 0.3 to 1 ppm |
| Potassium (K) | 3 to 10 ppm | 2 to 5 ppm |
| Magnesium (Mg) | 1 to 2 ppm in soft water | Only if your tap water is soft |
| Iron (Fe) | 0.1 to 0.2 ppm | 0.05 to 0.1 ppm |
High tech tanks with CO2 and strong light use plants’ full appetite, which is why Estimative Index (EI) dosing adds plenty and resets with a 50% weekly water change. Low tech tanks without CO2 grow much slower, so a quarter of EI or less is usually enough. Many low tech tanks also feed mostly through the roots, and our DIY root tabs guide covers that side.
How to make a stock solution
- Weigh the powder. Use a 0.01 g scale. Kitchen scales are not accurate enough for small amounts.
- Use distilled or RO water. Tap water can make some salts cloud up over time.
- Keep macros and micros apart. Mix KNO3, KH2PO4 and K2SO4 in one bottle and trace elements in another. Phosphate can bind with iron and drop out of the solution.
- Don’t overload the bottle. If powder stays at the bottom, add more water and recalculate.
- Label it. Write the recipe and the “ml per 1 ppm” number from the calculator on the bottle.
- Store it cool and dark. A few drops of an aquarium-safe preservative or a splash of liquid carbon helps stop mold in trace mixes.
Common fertilizer dosing problems
Algae after you start dosing. Usually light is outrunning everything else, not the fertilizer itself. Cut the light period by an hour or two before cutting nutrients. A cleanup crew of Amano shrimp and nerite snails also helps.
Yellow or pale new leaves. Often iron or nitrogen. Our plant deficiency symptoms guide helps you tell which nutrient is missing before you change the dose.
Plants melting after planting. That’s usually the plant switching from farm-grown leaves to underwater leaves, not a dosing issue. See our plants melting guide.
Shrimp or snails acting stressed. Copper and sudden big doses are the usual suspects. Dose small and steady in shrimp tanks, and check the copper line in the results table when using trace mixes.
Numbers don’t match your test kit. Hobby test kits are rough, and plants start using nutrients right away. Use the calculator for what you add and the test kit only to spot big trends.
FAQ
How much KNO3 should I add to my aquarium?
For EI dosing, most people add enough KNO3 for 5 to 7.5 ppm nitrate three times a week. In a 20 gallon tank that’s about 0.6 to 0.9 g per dose. Low tech tanks need far less.
What does ppm mean in aquarium fertilizers?
Parts per million. In water it’s the same as milligrams per liter, so 1 ppm of iron means 1 mg of iron in every liter of tank water.
Is EI dosing safe for fish and shrimp?
The normal EI levels are well below what’s harmful to fish. The 50% weekly water change is part of the method, so nutrients never build up. With shrimp, start lower and go up slowly.
Can I use this calculator for liquid fertilizers?
Yes, if you make your own liquid with the Stock solution tab. For bottled brand fertilizers, the label usually lists ppm per ml, so follow that.
Should I dose fertilizer in a tank without CO2?
Yes, but lightly. Plants in a low tech tank still need nutrients, just less of them. Start with about a quarter of EI and use root tabs for plants like swords and crypts. Our low light aquarium plants list shows which plants do well this way.
Why does the calculator show potassium when I picked nitrate?
Because potassium nitrate is part potassium. Every salt adds more than one nutrient, and the table shows all of them so you don’t overdose one by accident.