I've had a terrible time with algae on my little waterfall. It looked REALLY ugly. I quit using the waterfall last year, and now there's weeds growing out of all that algae! haha
So turning the water off to it didn't help.........even when I tried it intermittently before turning it off permanently. But I'm glad it worked for you!
My water isn't clear any more, and I'm realizing that in spite of the algae's ugliness on the waterfall, it was filtering the water really well. I guess I just can't have a nice-looking waterfall AND clear water. haha

I find 5% hydrogen peroxide from my local drug store works perfectly .. slow down but don't turn off the water then slowly pour an entire 8 or 10 oz bottle over the algae. I have to do it every year as the filter cycles and the water lettuce or water hyacynth are still getting established. Usually only takes one treatment.

Try looking into using a algaecide. There is a product called SeClear on the market that the people at SePRO suggested for waterfalls. Its used for ponds typically, but worked well for a waterfall too.

Here is the product.
http://www.lakelawnandpond.com/SeClear.aspx?ItemGroupID=61

Doesn't it make more sense to try to solve the cause of the excess nutrients so the string algae or suspended algae doesn't cause so many problems rather than dumping chemicals into our tiny ecosystems?

The problems are caused by a number of factors each of which should be investigated carefully. Is the bio-load(#/size of fish) too high, are the fish being fed too much, is there insufficient water movement/circulation, etc.

For me it simply came down to planting a few plants that would come out of their winter dormancy earlier than the algae. Completely unintentionally I placed a pot with 3 marsh marigolds in the pond mid-season. The marsh marigold began to grown when there was still ice on the pond and from that point on I have had almost no string algae and not 1 suspended algae bloom. Last fall I removed the filter to allow a contractor to pull an underground oil tank so this year we had some suspended solids which quilt batting removed but still no algae bloom of any kind.

Shalom - go Hydrogen peroxide - thought 3% was he standard retail concentration>? - even at 3% it works - need to add some patience into the mix

Yes it's 3% and effective. Stick with the H202 and leave the chemicals for those who don't mind draining their wallets. As rsingley pointed out, better to correct the underlying isssue than to treat symptoms..

@ catherinet, yes.you.can. have both! I don't recall you having a problem with string algae? Maybe I missed it. (I refuse to concede it might be my memory, lol & I did stay away for a bit). I used to wonder what was this stuff called string algae others were battling, until.... my tiny waterfall was suddenly covered with it, seemingly overnight. Never saw it comin'..

Fortunately, I was armed & dangerous thanks to the experienced ponders here. My pond was several yrs old by then so I had lots of ammo, lol. I shut off the water fall, doused it with H2O2, then waited a few minutes for it to do it's thing & rinsed with the garden hose. I did remove some by hand before rinsing & just tossed it around some nearby terrestrial plants. As you have discovered, it's full of nutrients & makes for great fertilizer. Never encountered it before or since in my pond but I've dealt with it in friends' ponds. A healthy balance is key to keeping it at bay.

Never did figure out why it appeared in my pond that one year as the pond was well established by then, well maintained & nothing had changed. I'm just thankful there's been no recurrence!

@ jacob12345: please kindly cease & desist from peddling your wares here.

blessings all around,
~digger

That is odd...that it didn't work for you catherinet. Was yours in direct sun? And String Algae? Maybe the month and the temps were in our favor. Though...sadly we miss it.

We were going to go they peroxide route...but I liked it. Only my husband had issues. Everyone loved it when they visited. So now...we miss it. A friend was sad that it was gone as well...figured may have to wait until next year.