Microbe-Lift Barley Straw Pellets work in much the same way as barley straw, only faster. Barley straw works by releasing certain natural chemicals as the lignin in the straw biodegrades. The barley extract works the same way only the breakdown compounds are released faster. Certain of these compounds react photochemically with sunlight and produce hydrogen peroxide, which is an oxidizing agent that also helps to keep the water clear. It is not necessary for the barley pellets or the straw to float on the top of the water for this reaction to occur. What is reacting with the sunlight is the dissolved organics that are released from the barley straw or pellets into the water column where it disperses pretty equally throughout the pond water.
Read more about algae control in ornamental ponds and using algaecides here.