I would think the movement from your waterfall would be enough to aerate as long as its constantly running.
I have a 90g w/a natural slate waterfall (pump is inside a filter box), and a pondmaster fountain at the opposite end. I have the fountain running all the time, but the waterfall is on a timer.
FYI Pleco's poop A LOT!!!!! So hope your filtration is up to that if you decided to add one. Me personally I wouldn't, but that's my opinion. If I was going to add an algae eater it would be snails.
I have used Microbe-lift Algaeway 5.4 before with good success. But if you decide to use any chemicals be VERY, VERY careful and make sure to read the directions. I am planning on adding a lot more plants this year. (My neighbor cut his tree down that was shading my pond last Fall - I had an algae problem from too much sunlight and am beginning to have it again now that it is getting warm here.)
ETA: Not sure how long you have kept your pond, but try to keep it free of organic debris such as leaves, they will decay and foul the water. I use a pool skimmer net. That plus a lot of sunlight can definitely be a cause to your algae.
Last edited by trukgirl; 03-17-2011 at 11:38 AM..