The short answer
A planted tank is stable when light, carbon and nutrients move at the same pace. Algae appears when light runs ahead of the other two. So the fix is almost never "more light" — it is usually less light, or more carbon and nutrients to match the light you already have.
| Setting | Low-tech (no CO2) | High-tech (injected CO2) |
|---|---|---|
| Photoperiod | 6–7 hours | 7–8 hours |
| PAR at substrate | 15–30 µmol/m²/s | 50–80 µmol/m²/s |
| CO2 | Ambient only | ~30 ppm during the photoperiod (drop checker: lime green) |
| Fertiliser | Lean, or root tabs only | Full macro + micro dosing (EI or similar) |
| Water change | 20–30% weekly | 30–50% weekly |
Do not mix the columns. High light with no CO2 and lean dosing is the single most reliable way to grow algae instead of plants.
Treat light as a dose, not a mood
Light is the accelerator. It sets how fast plants demand carbon and nutrients. Turn it up and everything else must keep up — or algae, which is far less fussy than plants, takes the surplus.
- Start low. Begin a new tank at 6 hours and moderate intensity. Increase only after you see new growth and your maintenance has become routine.
- Use a timer from day one. Plants respond to a consistent schedule; algae exploits an inconsistent one.
- A longer photoperiod is not a substitute for intensity. Running a weak light for 12 hours mostly grows algae.
- Keep direct sun off the glass. In this climate that matters more than most guides admit.
The siesta method — splitting the photoperiod into, say, 4 hours on, 3 hours off, 4 hours on — is sometimes used to slow algae in low-tech tanks. It works for some people because CO2 levels partially recover during the gap. In an injected tank it usually just wastes gas.
CO2 rewards steadiness, not volume
The target for an injected planted tank is roughly 30 ppm CO2 during the photoperiod. What matters more than the exact number is that it is stable. Swinging CO2 stresses plants, stresses fish, and hands algae the gaps.
- Start CO2 1–2 hours before the lights. The tank should already be at target when the photoperiod begins.
- Stop CO2 1 hour before lights out. After dark, plants stop consuming it — continuing to inject just drops pH and suffocates fish overnight.
- Read the drop checker as lime green. Blue means too little. Yellow means too much, and you are 30 minutes from a fish emergency.
- Keep surface movement. A sealed, mirror-still surface holds CO2 but starves the tank of oxygen. Fish gasping at the surface in the evening is the classic sign of too much CO2 combined with too little gas exchange.
If fish start gasping: turn the CO2 off immediately, increase surface agitation hard, and do a water change. Do not wait to see if it settles.
Fertiliser is not the villain
The instinct when algae appears is to starve the tank. It rarely works, because algae needs far less than plants do — you starve the plants first, they stop competing, and the algae wins.
Plants need macros (nitrogen, phosphorus, potassium) and micros (iron and trace elements) in proportion to the light and carbon you are giving them. The useful question is not "should I fertilise" but "is anything running out."
| What the leaves show | Likely deficiency |
|---|---|
| Older leaves yellowing first, plant looks pale overall | Nitrogen |
| Pinholes in older leaves, then rot | Potassium |
| Stunted, twisted new growth at the tips | Calcium, boron, or CO2 shortage |
| New leaves yellow with green veins | Iron |
| Dark green leaves, extremely slow growth | Not enough light or carbon — not a nutrient problem |
Root feeders (swords, crypts) benefit from root tabs. Column feeders (stems, mosses, anubias, buce) take what they need from the water. Most tanks want both.
Read algae like a diagnostic report
Algae is not one enemy. Each type points at a specific imbalance. Treating all of them the same way is why people end up dosing five products and fixing nothing.
| Algae | What it usually means | The actual fix |
|---|---|---|
| Brown diatoms | New tank, silicates, immature system | Wait. It fades on its own in weeks 2–6. |
| Green spot algae (hard dots on glass) | Low phosphate, or high light | Raise phosphate slightly. Scrape the glass. |
| Green dust algae | Young tank finding equilibrium | Leave it alone for 3 weeks, then wipe and change water. |
| Hair / thread algae | Excess light relative to CO2, or ammonia spikes | Reduce photoperiod, raise CO2, manually remove. |
| Black beard algae (BBA) | Unstable or insufficient CO2. Almost always. | Stabilise CO2 first. Spot-treat, then fix the cause — or it comes back. |
| Staghorn | Organic waste build-up, poor flow | Water changes, vacuum, improve circulation. |
| Cyanobacteria (blue-green slime) | Not algae — a bacterium. Low flow, high organics. | Fix circulation and organics. Blackout or treat, but the cause must change. |
Note the pattern: most of these are cured by stability, flow and water changes — not by a bottle.
The rule that fixes most planted tanks
Change one variable at a time, and give it three weeks. Most struggling planted tanks belong to people who adjusted light, CO2 and fertiliser in the same week, saw it get worse, and adjusted all three again. The tank never gets a chance to settle.
- Do not change light, CO2 and dosing in the same week.
- Trim damaged leaves before they rot in the tank — decaying plant matter feeds algae directly.
- Keep water changes consistent. In a high-tech tank they are not optional maintenance; they are part of the method.
Frequently asked questions
How many hours should aquarium lights be on for plants?
6–7 hours for a low-tech tank, 7–8 hours with injected CO2. Longer photoperiods do not compensate for weak light — they mostly grow algae.
Do I need CO2 for a planted tank?
No. Plenty of species — anubias, java fern, cryptocoryne, mosses, vallisneria — thrive without injected CO2. What you cannot do is run high light without CO2 and expect plants to win against algae.
How much CO2 does a planted tank need?
Roughly 30 ppm during the photoperiod, indicated by a lime-green drop checker. Yellow means you are approaching a level that can suffocate fish.
What causes black beard algae?
Unstable or insufficient CO2, nearly every time. Poor flow makes it worse. Spot treatments kill what is there, but it returns unless the CO2 is stabilised.
Will less light get rid of my algae?
Often, yes — because it lets carbon and nutrients catch up. Reducing the photoperiod by 1–2 hours is one of the cheapest and most effective first moves, and costs you nothing to try.
My fish are gasping at the surface in the evening. Why?
Almost certainly too much CO2 with too little surface agitation. Turn the CO2 off, increase surface movement immediately, and do a water change. Then re-tune, starting lower.
Related reading: how to cycle a new aquarium — an uncycled tank leaks ammonia, and ammonia triggers algae faster than any light setting. Also aquarium filtration basics, because flow is half of the CO2 problem.