Do you mean the Eheim Biopower filter?
I am running 2 Biopowers in my 180 litre tank, the 160 and the 240, and I do find that the small sponge right at the top clogs quickly. It is a fine sponge compared to the big one in the bottom basket, so it will clog quicker. But I am somewhat overstocked, due to closing down a 50 litre tank and moving the inhabitants into the 180 litre.
But having said that, it takes 3 to 4 weeks before I notice a significant loss of flow.
The 160 in an 85 litre tank should be fine. Do you have live plants in the tank which might be dying back a bit? Do you overfeed so the muck causing the drop in flow is uneaten food? I find the filters suck in the food quite quickly which is why I was turning them off for a couple of minutes after I put food in - until I forgot to turn them back on only found they were off next day. Do you have fish that poo a huge amount, eg plecs of any type?