Run one end of the tubing through the cabinet base and floor. Working from under the house is usually easier. Push a piece of stiff wire (a wire coat hanger works great) into the hole under the house and through the hole in the bottom of the cabinet. Insert the end of the wire into the end of the tubing and tape it in place. Good ol' duct tape works great for this. From under the sink, pull the wire and tubing through the holes. <p>From under the house, run the tubing from below the sink to below the aquarium. Push the end of the tubing up through the floor behind the tank. Secure the plastic tubing to the floor joist under the house to keep it out of the way. Wiring staples work well for this. Do not crimp the hose as this will impede the water flow. Do not puncture the tubing with the staples.
The photo above shows an in line water filter installed below a kitchen sink. Note that the saddle valve is installed above the cold water shut off valve.
