The short answer
Excess algae almost always comes down to one thing: an imbalance between light, nutrients and CO2. When there’s more light and nutrients available than your plants can use, algae takes the surplus. The usual culprits are light that’s too bright or on too long, overfeeding, and overstocking — often several at once. Fix the balance and the algae fades; chase symptoms and it keeps coming back.
The three levers: light, nutrients, CO2
Every algae outbreak traces back to these three being out of balance:
- Too much light. Long photoperiods, bright fixtures, or direct sunlight on the tank are the single most common trigger. Keep your light to 6–8 hours a day on a timer.
- Excess nutrients. Overfeeding leaves waste that becomes nitrate and phosphate; overstocking adds more waste than the tank can process. Both feed algae.
- Weak plant growth / low CO2. Healthy, fast-growing plants soak up nutrients before algae can. If plants struggle — often from unstable CO2 or missing fertilisers — algae fills the gap.
Common causes to check
Run through this list and you’ll usually find your trigger:
- Lights on too long, or too strong for a low-tech tank.
- Feeding more than fish clear in a couple of minutes.
- Too many fish for the tank volume — see how to tell if you’re overstocked.
- Skipped or infrequent water changes letting nitrate climb.
- A new tank that hasn’t stabilised yet.
Fix the balance
Trim the photoperiod, feed less, keep up weekly water changes, and get more healthy plants growing. Do those consistently and algae runs out of fuel. For a full plan see how to get rid of aquarium algae, and to understand light’s role, read does more light cause algae.