It's not effective if it floats on the water because only a small portion is touching the water. It will sink naturally. I use RED EARTH barley straw which is organic-it floats for about two or three days then sinks to the bottom where it does what it needs to do.

How long does barley straw take to sink?

This process can take anywhere from 3 weeks, to over 3 months, depending on water temperature and pond conditions.

Does barley straw sink in ponds?

Barley straw pellets are also available, but these sink to the bottom of the pond, and since they need to decompose aerobically (see below) I don't see how they can work. Barley straw works better than other types of straw, such as wheat straw.