I had the inline nozzle type air assist on my M40, but I sometimes found it a pain when aligning the beam to have it pass through the exact center of the nozzle. Now I have added air assist on my ULS-25 and I simply used a stainless steel tube with a needle point from one of those "canned air" sprayers. It works perfectly, and I don't have any fears about turning up the pressure or contaminants in the air hurting my lens. Concerning the lens, I also place a sacrificial window below my lens to keep it clean since windows are only $10 each. My system attaches to a full size DEWALT oil-less air compressor that I have in my garage, so I can go all the way to 120psi if needed. The full size DEWALT is much quieter than the little air compressors, but of course it is not as quiet as the aquarium pumps.
The nozzle is curved to point at the focus spot that is 2" below the lens. The tip of the nozzle rides about 3/4" above the workpiece.