To dose aquarium fertilizer correctly, work out your tank’s water volume in litres, follow the fertilizer label’s recommended millilitres per 100 litres, dose two to three times a week after a weekly water change, and always start at the lower dose before increasing. This simple, measured routine keeps nutrients steadily available so plants grow well without feeding algae. This guide shows you exactly how to calculate your dose, how to use liquid fertilizer step by step, a beginner-friendly schedule, and how to adjust over time written for planted tanks in India.
Dosing sounds technical, but it comes down to three numbers: your tank volume, the label dose, and how many times a week you dose. Get those right and the rest is easy.
Step 1: Know your tank’s water volume
Short answer: Use your real water volume usually about 85–90% of the tank’s rated size after substrate, rocks and the gap at the top.
Fertilizer doses are given per unit of water, so you need your actual water volume in litres. A “100-litre” tank rarely holds 100 litres of water once you account for substrate, hardscape and the few centimetres of air at the top it is usually closer to 85–90 litres. For a rectangular tank you can estimate litres as length × width × height (all in cm) ÷ 1000, then subtract roughly 10–15% for displacement. Knowing this number is the foundation of accurate dosing.
Step 2: Read the dose on the bottle
Every fertilizer states a dose, usually as millilitres per 50 or 100 litres, and how often to apply it. This is your starting point. Doses differ between products and between concentrated and regular formulas, so always dose to that product’s label rather than a number you remember from another bottle. If you use an all-in-one like Sunken Garden Green, follow its label for your tank size.
Step 3: Dose by tank size and tank type
Here is a typical starting guide for both low-tech and high-tech tanks. Always defer to your specific product’s label, this table is a general illustration of how dose scales with tank size and conditions.
Illustrative only always follow your fertilizer’s label. High-tech CO2 tanks dose more often and slightly heavier, and dense planting sits at the top of each range while sparse planting sits at the bottom. Not sure how a tank’s feet relate to litres? Our complete fertilizer guide and the dosing section there explain it.
Step 4: How to actually dose liquid fertilizer
- Do your weekly water change first (around 25% for low-tech, more for high-tech).
- Measure the dose using the bottle’s cap or a syringe, do not pour by eye.
- Add it to an area of gentle flow (near the filter outlet) so it disperses quickly.
- Dose into the water column; you do not need to turn off the filter.
- Repeat on your chosen days through the week.
For root tabs, the method is different you push them into the substrate near root-feeders. See root tabs vs liquid fertilizer.
Step 5: Start low and adjust
The single most important rule: start at the lower end of the dose and observe for two weeks. If plants grow well and algae is controlled, you are dosing correctly. If growth is slow or you see deficiency signs, increase gradually. If algae appears and growth is fine, ease back. This “observe and adjust” loop is how experienced aquascapers dial in their tanks. Learn to read the signs in signs of nutrient deficiency.
Target nutrient levels (for those who test)
If you own test kits, these are the water-column ranges healthy planted tanks typically sit in. Low-tech tanks do well at the lower end of each range; high-tech CO2 tanks aim for the upper end:
| Nutrient | Target range | Notes |
|---|---|---|
| Nitrate (NO3) | 10–25 ppm | Below ~5 ppm plants starve; well above 30 ppm favours algae |
| Phosphate (PO4) | 0.5–2 ppm | Zero phosphate stalls growth despite green water myths |
| Potassium (K) | 10–20 ppm | Deficiency shows as pinholes in older leaves |
| Iron (Fe) | 0.1–0.5 ppm | Deficiency shows as pale new leaves; dose micros/iron to correct |
You do not need to test to succeed the observe-and-adjust loop above works fine but these numbers are useful when troubleshooting stubborn deficiency or algae problems.
How often should you dose?
Most liquid fertilizers are dosed 2–3 times a week. Low-tech tanks (no CO2) sit at the lower end; high-tech CO2 tanks at the higher end, sometimes daily. The full timing breakdown is in when and how often to add fertilizer.
Dosing macros and micros separately
Advanced high-tech aquascapers sometimes dose macronutrients and micronutrients on alternate days, because some nutrients can interact if mixed in concentrated form in the tank at once. This is part of the Estimative Index method see EI dosing. For an all-in-one, this is not a concern; you simply dose the one bottle.
Common dosing mistakes
- Guessing the dose - always measure with a cap or syringe.
- Using the rated tank size, not water volume - subtract for substrate and hardscape.
- Dosing heavily from day one - start low to avoid algae.
- Skipping water changes - they reset nutrient levels and prevent build-up.
- Ignoring the plants - they tell you whether to dose more or less.
India-specific dosing tips
In India’s warm climate, plant metabolism rises in summer, so tanks may need slightly more fertilizer in the hot months, watch growth and adjust. Hard tap water already provides calcium and magnesium, so you are mainly dosing NPK and iron. A well-priced, locally-stocked all-in-one like Sunken Garden Green makes consistent weekly dosing affordable and easy to sustain.
Recommended for easy dosing
Frequently asked questions
How do I dose aquarium fertilizer?
Find your tank’s water volume in litres, follow the bottle’s dose per 100 litres, and dose 2–3 times a week after a weekly water change, starting at the lower dose.
How much fertilizer for my tank size?
Dose by water volume per the label. As a rough illustration, a 2 ft (~50 L) tank might take a couple of millilitres of all-in-one twice a week, but always follow the product label.
What nutrient levels should a planted tank aim for?
Roughly 10–25 ppm nitrate, 0.5–2 ppm phosphate, 10–20 ppm potassium and 0.1–0.5 ppm iron. Low-tech tanks sit at the lower end, high-tech CO2 tanks at the upper end.
How do I use liquid fertilizer in my aquarium?
Measure the dose, add it near the filter outlet so it disperses, and dose into the water column on your scheduled days. No need to turn off the filter.
Should I dose before or after a water change?
After. Do the water change first, then dose into the fresh water so levels start fresh for the week.
How do I calculate my tank’s water volume?
Length × width × height in cm ÷ 1000 gives litres, then subtract about 10–15% for substrate, hardscape and the gap at the top.
Can I overdose aquarium fertilizer?
Yes, heavy overdosing can stress livestock and feed algae. Measure carefully, start low, and do regular water changes.
How often should I dose?
Usually 2–3 times a week. Low-tech tanks dose less often; high-tech CO2 tanks more often, sometimes daily.
Do I dose every day?
Only high-tech CO2 tanks usually dose daily. Most low-tech and community tanks dose a few times a week.
What if I miss a dose?
Just resume your normal schedule, do not double up. Plants tolerate an occasional missed dose well.
Should I measure with the cap or a syringe?
Either, as long as you measure accurately. A syringe is most precise for small nano-tank doses.
Do root tabs get dosed the same way?
No — root tabs are pushed into the substrate near root-feeding plants, not added to the water. Replace them every few months.
How do I know if I am dosing enough?
Healthy new growth and no deficiency signs means you are dosing enough. Slow growth or pale/holey leaves means increase gradually.
Will more fertilizer make plants grow faster?
Only up to a point once nutrients are sufficient, growth is limited by light and CO2. Extra fertilizer beyond that just risks algae.
Can I dose an all-in-one and root tabs together?
Yes, dose the all-in-one into the water for water-column plants and use root tabs for root-feeders. They complement each other.
Is dosing different in a low-tech tank?
Yes, low-tech tanks grow slower and need less fertilizer, so dose less often and lighter than a high-tech CO2 tank.
Get your dosing right with a quality all-in-one from the Aquarium Fertilizer collection or the Sunken Garden range. Next, read when & how often to add fertilizer and the complete fertilizer guide.
0 comments