Home Made Filter Question

I have a 250 gal pond. 50 gal in the upper and 200 in the lower. My
pump is a 600 gal/hr pump with a bell fountain and the rest going to a
small water fall in the upper tank. It is 3 years old. I have 3
goldfish around 3-4", and probably 10 very small fish around 3/4" in
the lower pond along with snails and 3 tadpoles. I have a number of
plants in there. 2 nice sized lilly pads and some grass, and other
leafy airaters and sprauling hydrilla looking plants. Sorry I don't
know there names but they are all growing great. I built a filter with
3 plastic planting pots. In-between the 2 outer pots is 1/2 gravel.
In-between the 2 inner pots is small pebble sized lava rocks I
purchased from the local pond store. Inside the inner most pot, where
the pump is, I lined with washable filter material. The filter sits in
the lower pond. All the pots have many holes drilled in them and it is
cover on the top with the saucers from the pots with a hole drilled in
them for the bell fountain. It works great and the water is clear and
doesn't have an over abondence of blanket weed. Here is my problem.
The pressure goes down once a week and I have to take the filter
material out of the center and rinse it off. I have not yet put any
bio-filter boost in the pond yet fearing it will cause the pressure to
go down quicker. When the filter material is clear the pump works
great. After running the pump for about 3 months I took it apart and
the gravel and pebble lava rocks where completely clear of anything.
Maybe I should build a vacuum attachement and clean the bottom? Maybe
there something wrong with my design? Any thoughts would be greatly
appreciated?
Thanks
Pond-On
Travis