okay, i didn't check to see the type of pump you have. if it is external, then usually the pump housing is separate from the electric motor and is connected with a shaft to turn the impeller. the pump housing on a sequence pump is held on with six bolts i think, and they are torqued to a certain tension. there may be a gasket, so be careful. i cant remember. that said i used to clean out my sequence 4200 every spring a couple of times because the willow leaves are thin in the tree above, and they lay across the impeller blades layering until they clog the pump. all i had to do was clean them out and reassemble. the manufacturer can tell you the torque on a given pump. easy to do but takes a work bench and valving the pump so you can take it out without draining the pond. it this is a pebble then, it will be easy to find. the pump should be mounted with a strainer basket to catch small pebbles. if it is a submersible, i'm not familiar with their service requirement.