Pond scum is pretty much an ugly thing in the backyard pond and usually results from a complete unbalancing of the water system.
You’ll recognize it because of the floating algae that is coalescing into bigger chunks.
The trick is to fish this stuff out (put it on the compost pile)
And then get to work on the real problem, because if you don’t sort this out, it will simply reoccur.
Are you feeding the fish too much or too often?
Stop feeding the fish. I know it’s fun but they’ll do well enough for a bit on the algae itself it you leave them be. Did you fertilize the lawn and get some into the pond?
Excess fertilization will produce a massive algae bloom. The simplest thing here is to do a water change plus the other things below.
This can also happen after you feed the waterlilies. Do you have adequate pond coverage with plants?
We’re looking for approximately 30% of the pond covered by plants to reduce sunlight levels. Remember that sunlight will cause algae to bloom like crazy so we’re trying to limit this.
Do you have enough oxygenators in the water garden?
I generally recommend that you calculate the amount of uncovered footage and divide by three to give you the number of oxygenating plant bundles you need to turn the water clear. While this may sound like a lot, if you purchase the hardier ones (not tropicals) you’ll find that many of them will overwinter in deeper ponds and your problems will be a thing of the past.
It is really important to understand that there is a very fine line between healthy, clear water and green, soupy pond scum.
And that line is a balanced ecosystem in the pond. Without that balanced ecosystem, you’ll always have problems.
So get the plant coverage and oxygenating values right and your problems will disappear.
A reader writes saying she can’t afford to add all those plants.
Doug says: then you’ll have green water. 🙁
What about using pond chemicals to clean the water?
You can do this but understand that chemicals only mask the symptoms. As soon as the chemicals wear off, the problem will come back. So you can continue dosing with chemicals or you can fix the problem.
Doesn’t a pump produce oxygen in the water and stop algae from growing
Pond scum isn’t reduced by pond pumps, but if you over-oxygenate the water with too large a pump, then the pump is part of the problem.
Small pumps do indeed help oxygen levels and this is good. Most folks with small ponds use too large a pump because the enjoyment is in the water movement.
What about ultraviolet water treatment?
Pond UV lighting works really well particularly in fish ponds where plants get eaten by the fish.
You do have to run the ultraviolet (and the pond pump pushing water through it) 24/7 and it’s expensive with each bulb costing upwards of $100.
If your fish population isn’t high, it is cheaper to put in perennial oxygenating plants to control pond scum.
So the bottom line here is that pond scum can be controlled by all the systems of algae control.