We use sodium percarbonate to clean up the string algae in the spring. The first year we had to treat every few weeks pretty much through the summer. This year (year three of using it) we treated twice in the spring and have had no string algae since.
This is the same product that is packaged and sold as waterfall and rock cleaning products by pond supply companies. We buy it from a soap making supply company for a fraction of the cost. We just shut down the pump, sprinkle it on the algae, wait about 20 minutes and start the pump up again. Within 24 hours the algae is gone. The powder breaks down into hydrogen peroxide and soda ash when it comes in contact with the water.
If you have a lot of string algae you'll want to be ready with a net to scoop up the stuff that dies and floats loose right away. You do want to be careful not to leave a lot of dead and dying algae in the pond. When we first started using it, we would manually remove as much of the algae as we could before treating so there was less to scoop out after.
In early spring we sprinkle it liberally over the entire pond surface and it brings debris up from the pond floor to make it easy to scoop up and discard. We swear by the stuff - it's fish and plant safe, easy to use, and inexpensive when you buy it from non-pond related sources.
Our bog is our only filtration for our pond as well. Most of our water flows through the bog (we have a small side stream where we divert a small amount of water). We have a variable speed pump so we can change the water flow as desired.
Good work, by the way - looks great!