Various ways to get done what you want to do, but if it were me, this is what I would do.
Leave the manifold alone on the pump.
Just use one of the outlets and connect it to 3/4 black plastic water pipe via proper fittings. The black plastic pipe (like mentioned earlier in 500' rolls by Dnickolaus) is available in two different versions, the more expensive being higher pressure rating (can't remember for sure, but maybe one is 60 psi and the other is 100 psi, or something like that). Either one will work fine for your situation as you likely will only be using less than 10 psi pressure. They both are pretty cheap. You might want to spring for the thicker pipe just for better chew resistance, your call. Some plumbing supply stores may only carry one or the other. Lowe's carry 100' rolls of both types (it is not in the plumbing section, but in the well water pump section of the store). Probably have to go to a commercial plumbing supply to get the 500' rolls. More connectors if you go with the 100' rolls like I did because it was available locally. Bury this pipe from the compressor to the waters edge wherever you want your manifold. It does not have to be buried deep, or at all. But mice and critters love to chew on plastic. You could leave it lay on the surface till you get everything going, then simply dig a trench next to the line and throw it in.
At the pond's edge where you want to place the manifold make a little housing of some sort to hold the valve. You can make a valve setup out of quarter turn ball valves and associated fittings, but what I did was just get a good quality brass garden hose manifold. One of those where you attach to a faucet and it gives you multiple hose outlets. You can get them in two, three or four outlets. Use fittings needed to plumb in the 3/4" hose to the input side of this manifold. Use fittings to go from the outlet side to adapt to your sinking tubing going into the pond. I would use 3/8 if it were me unless it was a really long run from the pond edge to the diffuser. Then you might want to go larger. I did but don't think I really needed it after it was done, and in a later installation in another pond used the smaller 3/8 split out to two different diffusers. It all depends on air flow required and the distance but where you will be splitting the air (dividing the flow), unless it is over a hundred feet to each diffuser, I would think 3/8 would be fine. Some of the experts on here can advise if you need larger. A couple cubic feet is not much air to push through a line in a minutes time.
That is what I would do. Not that what I do is always right, so take it for what it is worth to you.
Last edited by snrub; 08/28/14 11:32 AM.
