mchambers
Former CCA member
With a pump in the tank, you're not going to get much, if any, removal of waste, because the pumps all have screens so that junk doesn't get in the impeller. When I want to remove waste from the bottom, I use the water changer attached to the faucet. I suppose you could somehow attach a pump to the output end of the water changer, and use the gravel vac that way, but I haven't found a way to do that. Also, I think you'd clog the pump.
As far as the length of the tube is concerned, I think the bigger issue is whether you are siphoning downhill or uphill. If the water is flowing level or downhill, any pump will do fine. If you're trying to send the water uphill, you need a more powerful pump. Pump manuals or specifications will tell how much lift (vertical rise) the pump can handle, and that will give you some idea of what you need.
As far as the length of the tube is concerned, I think the bigger issue is whether you are siphoning downhill or uphill. If the water is flowing level or downhill, any pump will do fine. If you're trying to send the water uphill, you need a more powerful pump. Pump manuals or specifications will tell how much lift (vertical rise) the pump can handle, and that will give you some idea of what you need.