How to Get Rid of String Algae in a Pond (Without Chemicals)

The natural way to get rid of string algae is to rake out the mats you can see, then reduce the excess nutrients that let them regrow, and keep the water moving with aeration. String algae (also called blanket weed or filamentous algae) forms the hair-like green tangles that float on the surface or cling to rocks and plants. It is a different problem from green, pea-soup water, so if your pond looks cloudy-green rather than stringy, start with our guide to getting rid of green pond algae naturally instead.

Green water vs string algae: which problem do you have?

These two look nothing alike once you know what to check. If you can grab a handful of green strands, lift a slimy green mat off the surface, or see hair-like tangles wrapped around rocks and plant stems, you have string algae, and this is the right guide. If instead the whole pond is a uniform cloudy green and the water itself looks tinted, that is planktonic green water, a separate issue we cover in our guide to getting rid of green pond algae naturally.

The distinction matters because the fix is different. You cannot rake out green water, and raking is the fastest first move against string algae. The two problems can also happen at the same time in the same pond, so it is worth checking for both.

What string (filamentous) algae is and why it forms mats

String algae is filamentous algae: long, thread-like cells that link end to end into strands, then knit together into the dense green mats you see floating or clinging along the edges. It often starts as a fuzzy green film on rocks, liner or plant stems, then lengthens and lifts off as it grows and traps oxygen bubbles.

Like all algae, it needs three things to thrive: sunlight, warmth and nutrients. Warm, sunlit, slow-moving water along shallow edges and shelves is where mats usually appear first, which is also why they tend to show up as summer sets in.

Why string algae keeps coming back: the nutrient loop

If your string algae will not go away, the reason is almost always that the nutrients feeding it are still in the pond. Fish waste, runoff, grass clippings, leaves and decaying plants load the water with the nutrients algae uses as food. Pull out a mat and leave the nutrient source untouched, and a fresh mat simply regrows on the same nutrients.

This is the loop to break. Physically removing algae gives you an immediate visible win, but lasting control comes from reducing the nutrient supply so there is less for the next generation of algae to feed on. That is why our approach pairs raking with beneficial bacteria and aeration rather than relying on removal alone.

Step 1: Rake the mats out physically

Start by removing as much of the visible mat as you can by hand. A mechanical weed cutter and rake, like the Cutter 'N' Rake, is designed to let you cut, twist and pull the strands out from shore, including mats wrapped around plants and along the edges that you cannot reach by hand.

Every mat you pull is a load of trapped nutrients you take out of the pond for good, so it is worth doing thoroughly. Lift the algae clear of the water, let it drip at the edge so any fish or invertebrates can slip back in, then compost it well away from the pond so it does not wash back in.

Raking does not make string algae spread. Fragments can regrow in the right conditions, so the goal is simply to remove as much as you can rather than to leave large pieces drifting; skimming up stray bits afterward keeps things tidy.

• Work from shore with a cutter and rake to reach mats around plants and along the edges.
• Lift algae clear, let it drain at the edge, then compost it away from the water.
• Skim up loose fragments so they do not resettle and regrow.

Step 2: Reduce the nutrients that fuel regrowth

With the bulk of the mat out, the next job is to shrink the nutrient supply so it grows back more slowly and less densely. Beneficial bacteria and enzyme conditioners digest the organic matter and consume the excess nutrients that algae feeds on, so the pond offers less fuel over time.

This is treating the cause rather than the symptom. Applied regularly through the warm months, beneficial bacteria also work on the bottom muck that acts as a nutrient reservoir, which is where a lot of string algae's food comes from. Reducing that reservoir is what gradually tips the balance in your favour season over season.

Step 3: Add oxygen and circulation with aeration

Aeration circulates and oxygenates the whole pond, which does two useful things. It supports the beneficial bacteria so they digest nutrients and muck more efficiently, and it keeps the water moving instead of leaving the warm, still pockets along edges and shelves where string algae mats settle in.

Bottom aeration, such as a windmill, solar or electric aerator, is often the most effective form because it moves water from the floor up. A surface fountain adds visible movement and aeration to the upper layer and looks good doing it. Together with bacteria, aeration is the engine that keeps the pond working against algae between rakes.

Why chemical-free removal beats the kill-sink-rot cycle

It is tempting to reach for a copper-based algaecide and watch the mats die back fast. The trouble is what happens next: the killed algae sinks, rots on the bottom and releases its stored nutrients back into the water, which can re-fuel the very growth you were trying to end. The visible algae is gone for a while, but the nutrient load, the actual cause, can go up.

Raking removes those nutrients from the pond entirely instead of recycling them, and bacteria plus aeration keep chipping away at what remains. Because the mechanical and biological approach never adds a chemical to the water, it is safe for fish, pets and wildlife when used as directed, which is the whole point of caring for a pond naturally.

A simple seasonal routine to keep mats from returning

String algae is easiest to manage when you stay a step ahead of it rather than waiting for a full mat. A light, repeatable routine through the warm season keeps it from taking hold.

This is the same Renew, Revive, Remove approach we apply across pond care: reduce nutrients, aerate, and remove what builds up. Use the dosing calculator to size beneficial bacteria to your pond, and get in touch if you want help matching an aerator to your water.

• Rake out visible mats as soon as they appear, before they thicken.
• Keep beneficial bacteria going through the warm months to reduce the nutrient supply.
• Run aeration to circulate and oxygenate the water and support the bacteria.
• Cut off inputs where you can: keep grass clippings, leaves and runoff out of the pond.