I assume you have an access port to the tank and this is diesel..
Here is what I did.. I vacuumed up the sludge.
Open the port, take a drill and an inexpensive pump (buy a couple..) that attaches to the drill and clear plastic hose about 5 feet on both sides attached to the pump. On the end that will go into the tank get 3-4 feet of aluminum tubing that will fit snugly on inside the clear hose. The other end where the fuel comes out of the hose..either lead it to empty jerry can or if the fuel is not too bad get a Baja filter and filter the crap out before pouring into the can and then the cleaned fuel back in the tank.
Helps if the tank is less than ˝ full so you can see the bottom. Have one person work the drill and monitor the contents going into the jerry can. The other person vacuums – they may need to train a flash light inside the tank to see what needs to be picked up.
Worked for me. Hope that helps . .
BTW.. if t you dispose of the diesel you can pour it into the recycle tank for oil at your marina.
ACM - Eastport MD