One of the most common questions I get asked is, “Why is my aquarium growing algae?” The first thing I tell people is don’t panic. Algae isn’t always a bad thing. In fact, every healthy aquarium will grow some algae. The problem starts when it takes over the glass, decorations, plants, rocks, or substrate and becomes harder to control than the fish themselves.
It’s Not About Being Dirty
One of the biggest misconceptions is that algae means your aquarium is dirty. That’s not always the case. I’ve seen crystal-clear tanks with algae covering every surface, and I’ve also seen older tanks with hardly any algae at all. Algae is usually a sign that something in the aquarium is out of balance, not necessarily that the tank hasn’t been cleaned.
Lighting Is a Major Cause
Lighting is one of the biggest causes. Aquarium lights left on for 10–12 hours a day, or tanks sitting in direct sunlight, are almost guaranteed to encourage algae growth. Fish don’t need bright lights all day long. The longer the lights stay on, the more opportunity algae has to grow. It’s one of the easiest problems to fix, but also one of the most overlooked.
Overfeeding Feeds the Algae
Overfeeding is another major contributor. Every bit of uneaten food eventually breaks down into waste, releasing nutrients like nitrates and phosphates into the water. Those nutrients don’t just disappear—they become food for algae. Fish need far less food than most people think, and feeding a little less is often healthier than feeding too much.
Water Quality Matters More Than You Think
Water quality also plays a huge role. High nitrate levels, poor circulation, dirty filter media, and organic waste trapped in the substrate all contribute to algae growth. Sometimes a tank can look perfectly clean from the front, but underneath the gravel there’s months of built-up waste slowly feeding the problem. This is one of the reasons regular maintenance is so important. It’s not just about making the tank look nice—it’s about removing excess nutrients before algae gets the chance to use them.
Different Algae Types Tell Different Stories
Different types of algae can also tell you a story about what’s happening inside the aquarium:
| Algae type | Usually means | What to do |
|---|---|---|
| Brown algae | New tank still maturing | Usually settles on its own as the tank establishes |
| Green algae | Excess light or nutrients | Cut lighting hours, reduce feeding, tidy up nutrients |
| Black Beard Algae | Something out of balance | Steady maintenance, stable CO₂ (planted tanks), better circulation |
Green algae is the most common and is often linked to excess light or nutrients. Black Beard Algae is one of the toughest to deal with and usually points to inconsistent maintenance, fluctuating CO₂ in planted tanks, or poor water circulation.
Don’t Rely on Chemicals
One thing I don’t recommend is relying on algae-killing chemicals as your first solution. They might remove what’s already there, but if the real cause hasn’t been fixed, the algae almost always comes back. It’s much better to solve the reason it’s growing than keep treating the symptoms.
Balance Is the Goal
In my experience, the healthiest aquariums aren’t completely algae-free—they’re balanced. A little algae is perfectly normal. The goal is keeping it under control by maintaining good filtration, avoiding overfeeding, managing your lighting, and staying consistent with maintenance.
If you’re constantly battling algae and can’t work out why it keeps coming back, Aquatics by Ash can help identify the real cause and get your aquarium back into balance, so you’re fixing the problem instead of fighting the same battle every few weeks.