An aquarium fertilizer calculator determines the correct amount of dry fertilizer salts to dose your planted tank based on volume and method. Whether you use the Estimative Index (EI) for high-tech tanks or PPS-Pro for low-tech setups, this calculator produces ready-to-use weekly schedules for KNO3, KH2PO4, K2SO4, and trace elements.
Tank & Method Setup
Fertilizers to Include
How to Use the Aquarium Fertilizer Calculator
Dry fertilizer dosing is the most economical and precise way to fertilize a planted aquarium. Unlike liquid fertilizers that come pre-diluted, dry salts let you control exact concentrations for each nutrient. This aquarium fertilizer calculator generates a complete weekly schedule based on your tank volume and preferred dosing method.
Step 1: Choose Your Dosing Method
The Estimative Index (EI) is the most popular method for high-tech tanks with CO2 injection and high light. It uses generous doses 3 times per week with a 50% water change on Sunday to reset accumulated nutrients. EI eliminates nutrient deficiencies as a variable, making it easier to diagnose and fix algae problems. PPS-Pro doses daily in smaller amounts, aiming to maintain near-zero residual nutrients. It suits low-tech or non-CO2 tanks and reduces water change frequency.
Step 2: Enter Tank Volume
Enter your net water volume — not the tank's total capacity. Subtract roughly 10–15% from the tank's rated volume to account for substrate, decorations, and equipment. For a 20-gallon tank, a usable water volume of 17–18 gallons is typical. Use our fish tank volume calculator if you need help determining exact volume.
Step 3: Select Your Fertilizers
KNO3 (potassium nitrate) provides nitrogen and potassium. KH2PO4 (monopotassium phosphate) provides phosphorus. K2SO4 (potassium sulfate) supplements potassium if your plants show deficiency without adding more nitrate or phosphate. Trace mix (typically CSM+B or similar) supplies iron, manganese, zinc, boron, copper, and molybdenum. Most planted tanks dose all four.
Step 4: Mix Into Solution
Prepare separate stock solutions for macros and trace elements. Dissolve your weekly dose of macro salts (KNO3, KH2PO4, K2SO4) into 500 mL of reverse osmosis or distilled water. Store in a labeled bottle in the refrigerator. Prepare trace mix separately as iron can react with phosphate if mixed. Dose daily by adding a portion of the stock solution directly to the tank.
Step 5: Monitor and Adjust
With EI, if plants show deficiency symptoms (yellowing, holes in leaves, stunted growth), increase the affected nutrient's dose by 25% the following week. With PPS-Pro, monitor phosphate and nitrate directly with test kits — target near-zero residual levels. In both cases, give changes 2 full weeks before evaluating results, as plant nutrient responses are slow and incremental.
Safety and Fish Compatibility
The doses produced by this calculator are safe for fish and invertebrates at recommended tank volumes. However, very sensitive shrimp (Crystal Red, Crystal Black) can be sensitive to sudden chemistry changes. When starting a new fertilizer regimen with shrimp tanks, begin at 50% of the calculated dose and ramp up over 2–3 weeks. Always ensure adequate water flow so fertilizers don't concentrate in stagnant areas.
FAQ
Is this aquarium fertilizer calculator free?
Yes, this aquarium fertilizer calculator is completely free with no signup or account required. All dose calculations run locally in your browser — your tank data is never sent to any server.
Is my data private?
Absolutely. Everything runs client-side using JavaScript. Your tank volume and nutrient parameters never leave your device. No tracking, no cookies, no server-side storage.
What is the Estimative Index (EI) method?
The Estimative Index, developed by Tom Barr, is a high-dosing fertilizer method that saturates the tank with nutrients several times per week, then uses a 50% water change on weekends to reset levels. EI removes the guesswork from nutrient deficiencies and works best in high-light, high-CO2 tanks with fast-growing plants.
What is the PPS-Pro method?
PPS-Pro (Perpetual Preservation System Professional) is a low-waste daily dosing method that aims to maintain stable, near-zero residual nutrients. You dose small amounts each day to replace only what plants consume. It suits low-tech, non-CO2 tanks and tanks where you prefer weekly water changes of 10–20% instead of 50%.
What do KNO3, KH2PO4, K2SO4 provide?
KNO3 (potassium nitrate) provides nitrogen and potassium. KH2PO4 (monopotassium phosphate) provides phosphorus and potassium. K2SO4 (potassium sulfate) adds extra potassium and sulfur without adding nitrogen or phosphorus. Together these three macro salts cover all major plant macronutrients except calcium and magnesium, which are typically supplied by tap water.
Where can I buy dry aquarium fertilizers?
Dry fertilizer salts are widely available from aquatic plant vendors and general hydroponic suppliers. Common sources include GreenLeafAquatics, NilocG, and Amazon. Buying in bulk (100–500 grams) is much more economical than liquid fertilizers and will last months for most tanks.
Do I need to do water changes with EI dosing?
Yes, water changes are essential with EI dosing. The 50% weekly water change prevents nutrient accumulation, removes excess nitrate and phosphate, resets the system, and prevents algae from exploiting nutrient imbalances. Skipping water changes with EI typically leads to algae outbreaks within 2–3 weeks.
How do I mix dry fertilizers into a liquid solution?
Dissolve your weekly dose of each salt in 500 mL of RO or distilled water separately, then dose each solution daily (divide by 7 for EI, dose the full solution daily for PPS-Pro). Label each bottle clearly. Prepared solutions should be refrigerated and used within 2–4 weeks to prevent bacterial growth.