What size is the barbed fitting? Where can I find this part in the correct size? Please help.

I got this answer straight from the Manufacturer: The AAPA110L has a fitting with a G1/4" Thread size and a 10mm (3/8)" hose barb size. The important thing to note here is the "G", this is not NPT (which is Tapered). The G standard in non-tapered, so NPT will not fit.

Is it 25 L/min per each outlet or is it 25 L/min total?

25L/min total. All 8 outlets are connected through one manifold.

Does this unit include airline tubing and/or stones?

No not at all. Make sure you get a semi wide tubing. Also use both outputs other wise its loud.....but other than that its amazing

If I choose to use only 1 outlet at a time. Does it turn the other outlet off and only 1 outlet pumps air? Do I have to use 2 outlets at same time

According to the customer pictures of the inside of one there are two separate pumps inside the housing, each running to one line. So one pump should not affect the other. You should be able to place them at different depths, run different lengths of tubing and even leave one completely un-connected. It'll still pump out air, but no negative affects.