I am not sure what sheet algae is, is it the same as string algae? I have had string algae several times over the years. I took apart my entire pond and scrubbed every flat surface. Then reassembled and kept debrie from falling into it as light as possible with daily cleanings with a net. Only takes a minutes when I feed my fish and helps a great deal. My pond is under a wheeping cherry and gets much leaves and blooming petal fall. Last fall I covered it with a net as soon as leaves began to fall, and kept it covered with the net all winter as I get a great deal of winter fall/blow into it. The net reduced it by ten fold...I will never enter another fall/winter without netting it again. Is a net an option? Once you rid the pond of the algae, use barley straw, it comes in either sheets you can slide under plants or rocks or in pellet form. I liked the pellet form best. If it is string algae, hand removel is about the only way I know of to remove it or so I have been told. And it will continue to regrow as fast as you remove it if you do not find and fix the source. I use the barley and a NET during times of falling leaves and I have not had string algae in about five years now...knock on wood..there is not much worse as a pond owner...good luck!

The Algae are in big sheets. Bright green and sort of like a wet tissue paper but with more body. (A very scientific description I know) I've netted it out and they come out in maybe 12 inch by 12 inch pieces.
I always thought barley straw was for mosquitos.

I have a small pond, probably 3 x 6 but it's deeper than yours, about 3 feet. I had a algae about 3 yrs ago, the people on the pond forumn suggested using hydrogen proxide, I did and the problem cleared up and I continue to use it and the algae never came back. I clean the pond in the spring put in an 8oz bottle of hydrogen proxide, if the water starts to get cloudy i drain part of the pond and pour in about a half a bottle and refill the pond.