I ordered one and it just came in yesterday, what is the white filter bag with the draw string? Is that the muck bag ?
Yes.
Wrap the drawstring around the hose and secure it with a half hitch or two, or the bag tend to come off.
Is it placed over the discharge to filter what might go back into the pond if you were discharging into the pond?
Yes, though fines will go through it. Oase intends that the muck bag be kept on dry land. I put the bag in the pond in one spot, vac the rest of the pond, and then take the discharge out of the pond and vac the spot where the bag was. Recently, I've been experimenting with using quilt batting to trap more of the fines, but I haven't found a good configuration yet.
I used the vac last night without the bag and discharged the pond water onto near by rose bushes.
If you don't mind the pile of muck that builds up at the discharge, that's teh way to do it.
I ran a garden hose into the pond to refill as i vaccum.
If you want to do a water change, wait until you're done vacuuming to refill. This will give you the greatest reduction in pollutants. If the water level in my pond gets too low while I'm using the vac, I just turn the pump off until I'm done.