I would use 3/4" black plastic water pipe that comes in rolls. It comes in two different pressure grades. Either would be fine for your application but the higher pressure grade has thicker walls, is a little less prone to kink, and might take longer for a rodent to chew through. Price difference is minimal. Big box stores carry it in 100 ft rolls and it is very cheap compared to other types of tubing.
It is in the department that has home well pumps and tanks, not in the plumbing area. Buy the fittings made specifically for it (same area as pipe) as they are longer than normal. In cold weather slightly heating the pipe end will facilitate getting the fitting in the stiff pipe. If you connect two 100 ft sections together, double hose clamp each side. You do not want expansion/contraction pulling the joint apart and have an air leak.
Some of my aeration escapade. I burried the described water pipe to the pond then sinking air line to the diffuser. I used the water pipe to run part way in the water but that is a mistake. A pain in the rear to keep the water pipe on the bottom of the pond. Use sinking in the water.
initial installation installing additional aeration stuff for more ponds 
