All the crud goes up one end of the tank

Hi All,

Just wondering if anyone else has this problem or knows how to deal with
it. The problem only appears to occur in one tank - the 4 foot
(47.5gall) Malawi. Tank has a Fluval 304, Air Pump and a recently added
Fluval 4 Plus (put in to deal with an overstocking issue due to the
number of births/survivors). The substrate is a white marine sand/fine
gravel. Lots of Ocean rock and just one plant that is clinging on (Anubia).

On one side of the tank the gravel is glistening. The other end of the
tank it is covered in crud. The end with the crud is the end were the
outlet is for the 304 and the 4 plus is now positioned. I regularily
decrud the filter medium in tank water and concentrate my gravel vac in
that area but it comes back after 30 mins are so. I'm guessing it must
relate to currents in the water pushing it back to the same place but I
have the same set up in my other 47.5 gall tank and don't see the same
issue....

Any ideas? I don't think that the fish care too much as long as I keep
up the cleaning/water changes to prevent any toxins building up...but it
does make the whole thing look a bit of a mess.

TIA
Gill