If the bubbles are captured higher up, say 500m above the tank, so that the increase in deplaced water can lift the tank, you couldn't lift the tank all the way

If the bubbles are captured higher up, say 500m above the tank, so that the increase in deplaced water can lift the tank, you couldn't lift the tank all the way up to the surface; it would be stuck at 500m below surface. It's the basic law of conservation of energy. You need to provide the energy required to lift the tank by compressing the air. The energy required to lift the tank is mgh (mass of tank, gravitational constant, height to lift the tank). The energy stored inside the tank is proportional to p*V (pressure times volume). You can't lift the tank further than the amount of energy you have available, so you can never have a surplus of energy.