what's needed:
a container ( a 5 gallon bucket will do but it can be bigger if you want)
a power head (or you can use your overflows from the DT)
sand
how to set up:
drill 2 holes on the side of the bucket, at the top and one in each side.
attach a hose for water input on one side and another for the output.
fill the bucket with sand about 1/3 to 3/4 full.
and you're done.
how it works:
water coming from your sump feed by a pump (or from your DT) will pass at the surface of the sand in the bucket and will overflow back to your sump. it will take a while untill you will see result because the sand needs to build up bacterias first. but once the sand is established you will see the difference. I had this and it works wonders.
NOTE:
make sure that the sand in your RDSB is not disturb and the water that passes through it is just a stream not high flow.