Hi Mr Big
Firstly, is it possible for you to post pics of your setup?
Then, how big is your pump, I suspect that if you have debris etc sitting at the bottom of your pond that you are perhaps not creating a current strong enough into your settlement chambers.
Another thing I would suggest you do is perhaps move the curlers etc into your 2nd chamber, your 2nd chamber should effectively be your bio chamber, as altho your shells would be assisting with this, it could do with the assitance of an alternative media controlling the bio activity........perhaps try and source brushes and place those instead into your first chamber.
How does your water also return to the pond? If you are finding your debris is settling in specific spot and does not direct toward the bottom drains then you would need to modify your returns to enable such movement within the pond or else add in airstones into these dead areas that this debris settles in to cause the movement you require.
Stones at the bottom I would just say is a bad idea, once you get your flows sorted out and you are turning your water over effectively and feeding the right food you will have water that allows you to see a anything and everything on the pond floor it will be that clear.
I really suspect that you pump may be incapable at the moment and that you are possibly not turning your water over enough and that you are sitting with way too many dead areas within your pond.
Where about are you situated and could you post pics? If you have an issue posting you can always email me and I will post theem for you.... cliff.fagan@gmail.com
One more thing, is if you say your pond is 6 meters deep, where the shallow end is 2m deep and it's only 22000 litres then I suspect this pond's surface area must be very small?