Tom,
Unless you have a large pond, you wouldn't be running 300' in the pond. As I recall, you asked earlier about a run of a few hundred feet up over your dam. Just pointing out that the run TO your pond can be done with simple poly tubing like they use for irrigation/landscaping. That is much cheaper than weighted tubing. You can get 5/8 or 3/4" poly tubing for cheap in 500' rolls. Put a manifold/valve distribution box together at your pond, then run weighted tubing into the pond and to your diffuser locations. For a simple setup where you're 2 CFM per diffuser, the 1/2" weighted tubing Sue recommends won't have a large pressure drop for a hundred feet or less. I would do the pressure drop calc if I had >2 CFM or >100' per leg.
