Since you have no substrate you can use this idea. Get a section of hose that will reach the bottom of the tank and attach it to the intake on a good sized powerhead. Attach another length of hose, as long as you need to send the water where you want it to go, and attach it to the output of the power head. As long as you just have detritus to vac up, you shouldn't have a problem. I've used this to drain tanks that I had sitting on the floor.