Why does my aquarium turn green so quickly?

Every aquarium enthusiast knows the feeling: you’ve just cleaned your tank, and only a few days later a green haze starts to appear on the glass or your plants. Frustrating! But why does this happen? The answer lies in an imbalance within your aquarium’s ecosystem.

Do you think your aquarium might be out of balance too? Here’s how to fix it for good!

1. Too much light — The driving force behind algae

f your aquarium is exposed to direct sunlight or your artificial lighting stays on for too long, you’re giving algae the perfect opportunity to take over.

Tip: Avoid direct sunlight and use a timer for your lights. This helps you mimic a natural day–night cycle and keeps your aquarium balanced.

2. Too many nutrients = too much waste

Overfeeding is one of the most common mistakes. Leftover fish food breaks down into waste products like phosphates and nitrates — a feast for algae.

Rule: Everything you feed should be eaten within 5 minutes.
Better: Feed 2 to 3 small portions per day instead of one large meal.
Extra tip: Skipping feeding one day a week is healthy for your fish and helps keep the water clean.

3. Poor water circulation

Without enough oxygen in the water, dead zones can form where waste accumulates. Solution: Let your filter gently ripple the water’s surface to create small waves. This improves oxygen exchange and keeps your fish more active and healthy.

4. Too few plants = more room for algae

Plants and algae compete for light and nutrients. The more healthy plants you have, the less chance algae have to take over.

Tip: Choose fast-growing plants to outcompete algae.
Bonus: Plants naturally reduce nitrate and phosphate levels in your water.

5. Too many fish

The more fish you have, the more waste is produced — and that means more food for algae.

Advice: Keep an eye on your tank’s stocking level. Fewer fish = a healthier ecosystem.

A classic rule of thumb is 1 cm of fish per liter of water, but this only applies to small, slim species like guppies, neons, or rasboras.
For medium-sized fish (such as swordtails or gouramis), use 1 cm of fish per 2 liters of water.
For larger species (like cichlids or goldfish), it’s best to allow 1 cm of fish per 4–5 liters of water.

6. Maintenance: the key to clear water

A well-maintained aquarium stays clean for much longer.
Clean your filter regularly, but not too thoroughly — you want to keep those beneficial bacteria alive.
Occasionally vacuum the substrate to remove built-up debris.

The perfect balance = crystal-clear water

A green aquarium isn’t a mystery — it’s a sign of imbalance. By fine-tuning the balance between light, nutrients, plants, and maintenance, you create a stable ecosystem where algae hardly stand a chance.

With a bit of care and routine, you’ll enjoy a crystal-clear aquarium full of vibrant, healthy fish — just the way it should be!