Even if you got a siphon to fill it which is very possible, there would be no way to circulate the water through it. Also the water level in the bio tower would only be t the level of the tank and wouldn't make it over the rim.you could prob do this with out a pump if you think about it no extra electric just syphon it
You could possibly get a siphon to start in both the output and intake tubes if you design them right but you still need something to move the water.