Why algae grows in the first place
Algae is always present in spore form; it only blooms when conditions favour it over your plants. The trigger is an imbalance between light, nutrients and CO2. When there is more light and nutrients available than your plants can consume, algae — which is far less fussy than higher plants — takes up the slack. That is why the fastest-growing tanks with vigorous, healthy plants tend to have the least algae: the plants win the competition.
The most common sources of the excess are simple: too long a lighting period, direct sunlight on the glass, overfeeding, overstocking, and letting nutrients build up between water changes. Sort those out and you starve the algae at the root.
Step 1: Cut the light
Light is the biggest lever. Put your aquarium light on a timer and hold the photoperiod to 6–8 hours a day. Move the tank out of direct sunlight, which is intense and impossible to control. If you are running a bright light over a low-tech tank with no CO2, dial the intensity down — high light without matching CO2 is a classic algae recipe. Our best planted-tank light guide covers controllable options.
Step 2: Reduce the nutrient load
- Feed less. Uneaten food is the number-one nutrient source. Give only what fish clear in a couple of minutes, once or twice a day.
- Check your stocking. An overstocked tank produces more waste than plants and filtration can handle. See how many fish you can keep.
- Change water regularly. Weekly 25–30% water changes export the nitrate and phosphate algae feed on.
- Test your source water. Some tap water is high in nitrate or phosphate before it even reaches the tank.
Step 3: Get plants growing
Counter-intuitively, adding more healthy plants often cures algae. Fast-growing stem plants and floating plants are nutrient sponges that outcompete algae directly. Give them what they need to thrive — sensible light, an all-in-one fertiliser, and CO2 if you run high light. A planted tank in balance simply does not leave enough spare nutrients for algae to exploit. If you are just starting out, our aquascaping guide covers reliable beginner plants.
Step 4: Manual removal and a clean-up crew
While the balance settles, physically remove what you can: scrape the glass with a magnet cleaner or blade, twirl thread algae out with a toothbrush, and siphon out loose material during water changes. Then let a small clean-up crew keep the finish tidy — Amano shrimp, nerite snails and Otocinclus are gentle, effective grazers. Remember they are a supplement to good husbandry, never a substitute for it.
Identifying common types
Different algae hint at different causes. Green spot algae on glass often points to low phosphate or simply time between cleans. Brown diatoms are almost universal in new tanks and usually fade on their own as the tank matures. Black beard algae tends to follow unstable CO2 and heavy flow onto hardscape. Stringy green hair algae signals excess light and nutrients. In every case the fix is the same underlying trio: rein in light, trim nutrients, and let strong plant growth take over.