Dry fertilizers vs liquid: cost over a year

Liquid fertilizers cost 10-20x more per dose than mixing your own from dry salts. The math, the tradeoffs, and whether the convenience is worth it.

A 500 ml bottle of Seachem Flourish Comprehensive costs $10-15 and doses a 200-liter planted tank for about 2-3 months. A 1 lb bag of potassium nitrate costs $6 and, mixed into solution, doses that same tank for over a year. The price difference between commercial liquid fertilizers and dry salts is enormous, and the chemistry is identical.

What you're paying for with liquid

Commercial liquid fertilizers (Seachem Flourish and similar all-in-one products) are pre-mixed solutions of the same salts you'd use in dry form: potassium nitrate, monopotassium phosphate, potassium sulfate, chelated iron, and trace elements. The manufacturer dissolved them in water, added stabilizers and chelators, and bottled them.

You're paying for the mixing, the bottle, the label, and the convenience of dosing with a cap rather than a scale. For small tanks and casual planted setups, the convenience has value. For larger tanks or serious planted tank hobbyists, the cost difference adds up fast.

Cost comparison: one year, 200-liter tank

Estimative Index (EI) feeding with liquid products:

• Seachem Flourish Comprehensive (trace): ~$12/bottle, lasts ~3 months = $48/year
• Seachem Flourish Nitrogen: ~$10/bottle, lasts ~2 months = $60/year
• Seachem Flourish Phosphorus: ~$10/bottle, lasts ~3 months = $40/year
• Seachem Flourish Potassium: ~$10/bottle, lasts ~2 months = $60/year
• Total: roughly $200/year

EI feeding with dry salts:

• Potassium nitrate (KNO3), 1 lb: $6, lasts 12+ months
• Monopotassium phosphate (KH2PO4), 1 lb: $6, lasts 24+ months
• Potassium sulfate (K2SO4), 1 lb: $6, lasts 12+ months
• CSM+B trace mix, 1 lb: $12, lasts 12+ months
• Total: roughly $30/year, and you'll have leftover salts going into year two.

That's a 6-7x difference. For a larger tank (400+ liters) or a fishroom with multiple tanks, the savings are even more dramatic. The dosing calculator computes exact amounts per addition for both Estimative Index and PPS-Pro methods using dry salts.

How to mix dry salts

You have two options:

Dry dosing. Weigh the dry powder on a small digital scale (0.01g resolution, $10-15) and add it directly to the tank. Simple, no mixing solutions, no bottles to store. Downside: small doses of trace elements are hard to measure accurately in dry form. A typical trace amount for a 200-liter tank is 0.1-0.3 grams, which is at the limit of cheap scales.

Stock solutions. Dissolve a measured amount of each salt into a bottle of distilled or RO water to create a concentrated liquid. Then dose by volume (ml) using a syringe or measuring cup. This is essentially what the commercial products are, except you made it yourself at 1/10 the price. Stock solutions of macronutrients (KNO3, KH2PO4, K2SO4) last indefinitely. Trace element stock solutions should be used within 2-3 months because chelated iron degrades over time.

A common recipe for an Estimative Index stock solution:

• 60g KNO3 dissolved in 500 ml water: add 10 ml three times a week for a 200-liter tank
• 10g KH2PO4 dissolved in 500 ml water: add 10 ml three times a week
• CSM+B: 30g dissolved in 500 ml water: add 10 ml three times a week (alternate days from macros)

The exact amounts depend on your tank volume and target concentrations. The dosing calculator generates the stock solution recipe for your specific tank.

When liquid makes sense

Liquid fertilizers aren't a ripoff for everyone. They make sense when:

• You have one small tank (under 60 liters) and feed infrequently. A single bottle lasts months and the annual cost is under $30.
• You don't want to buy a scale or mix solutions. The convenience is worth $150/year to some people.
• You're running a low-tech tank with slow-growing plants that need minimal supplementation. A bottle of all-in-one dosed once a week at half strength is easy.

They stop making sense when you have a large tank, multiple tanks, or a high-tech setup running EI dosing on a regular schedule. At that point, switching to dry salts pays for the scale and mixing supplies within the first month.

Where to buy dry salts

Aquarium-specific vendors sell pre-packaged EI feeding kits with all the salts for $20-30. This is the easiest entry point. For bulk purchasing, the same salts are available from chemical supply companies or agricultural fertilizer suppliers at even lower prices, though minimum order quantities are often 5-25 lbs per type.

The salts to look for by name: potassium nitrate (KNO3), monopotassium phosphate (KH2PO4), potassium sulfate (K2SO4), and CSM+B. CSM+B is a standard micronutrient blend containing iron, manganese, zinc, copper, boron, and molybdenum in chelated form.

Store dry salts in sealed containers in a cool, dry place. They don't expire. A single purchase typically lasts 1-2 years for a single tank.

Making your own liquid concentrates from dry salts

The cost advantage of dry salts is largest when you mix concentrated stock solutions that you dose from daily, rather than measuring dry powder each time.

Two-bottle method: Mix separate concentrate bottles because some salts precipitate if combined at high concentration. Bottle A: calcium nitrate and iron chelate in 500 mL of distilled water. Bottle B: potassium sulfate, magnesium sulfate, and monopotassium phosphate in 500 mL of distilled water. Dose equal volumes from each bottle into your reservoir.

Measuring precision. A $15 jewelry scale that reads to 0.01 grams is essential for weighing the dry powders. Kitchen scales that read to 1 gram are too imprecise for trace elements where a fraction of a gram makes a real difference in concentration. Weigh each salt individually, add to the stock solution bottle, shake until dissolved, and label clearly.

Shelf life of concentrates. Stock solutions stored in sealed containers in a cool, dark place last 2-4 weeks. Iron chelate solutions degrade faster under light exposure. If you see sediment or crystals forming at the bottom of a concentrate bottle, the salts are precipitating and the solution is no longer accurate. Mix fresh.

The nutrient mixing calculator generates recipes for concentrate bottles based on your target PPM and reservoir volume.