Is the tanks bottom above the floor? If so then:
Gets some garden hose and cut off approx 1-2m long enough to reach and fill a bucket of the floor whilst end is in the bottom of the tank. Get some 16/22mm tubing approximately 7cm (I got mine from my LFS when I asked for a sample to test size they gave me an off cut for free) and a hose clamp to clamp the two hoses together (£1). Attach the larger pipe to one end of the hose extending the end, you now have a gravel vacuum to get the flow started suck then end of the hose with the suction end below the end in the tank or do the same but use a turkey baster to create the suction or submerge the pipe and allow to fill with water, seal up both ends with your fingers and then release when the end you want water to come out of is below the other outlet preferably in a bucket and the flow will start.