My cold water tank, has one inch of brown sludge at the bottom, what should I do
It is hard to say what the sludge is without examining it.  If light is able to get in and if the water stagnates alot instead of flowing, it could be organic.  If that is the case, I would boost the chlorine level.  Also, a water softener would reduce the mineral content of the water, which would reduce all sludge, organic or not.  Also, check the pH of your water to make sure it is not too acidic or basic.  If so report to your water supplier utility.  My understanding is they are supposed to provide neutral pH water.  If they don't remedy, add appropriate chemicals to neutralize pH.

If you can detach and remove tank, do so.  Once detached, If you can get a filler brush in there go ahead and brush off the bottom.  Get some vinegar and baking soda or any commercial grade non-toxic solvent (Puriclean or Milton) in there and agitate the tank.  Back wash several times with hot water and repeat procedure  until all sludge is gone.

For in situ situations, here are how different people did it at this forum:

If you can get the end of a hosepipe through the inlet, with a high pressure spray fitting on the end, just wiggle and poke it around and then mop out the residue with a rag/hand pump. 

or

a stick with a ball of cloth on the end then lots of rinsing. 

or

by putting a little sharp sand in plus some water and sloshing it all about