If you have Blanketweed it is better to try to pull most of this out beforehand. You can remove a lot of Blanketweed by twisting a stick or cane into the weed as it sits on the pond sides.
The pond vac is designed to pick up stones and debris particles up to 8mm. Using the open nozzle will allow larger particles to pass through. The stones shouldn’t go into the black chamber but they can build up. Remove the impeller chamber, shake it and the stones should drop out.
Check the hose adaptor by removing the viewing chamber and shake out as stones can sometimes get lodged here.