In my years of watercooling, I've used a good number of brass fittings, as well as nickel plated brass fittings, and I've never had any severe oxidation/corrosion problems when mixed with a copper loop.

I mean, look at radiators - most radiator cores are made of brass with copper fins attached to them, and people rarely have problems with oxidation/corrosion inside the radiator.

After cleaning off the oxidation that you have experienced, what you really need to concentrate on is using the correct liquid for your WC system - a lot of the pre-mixed watercooling-specific liquids are made to look pretty, and they advertise this and that about preventing growth and corrosion, but more often than not, this tends to be a load of crap from the manufacturer.

I've been watercooling since 2003, and have never used anything outside of 90% distilled water, 10% antifreeze, and a few drops of UV dye. This will prevent any growth within the system, and antifreeze is a great anti-corrosive agent that will protect any system with mixed metals (even if its only copper/brass in the same loop). Antifreeze is used in cars with steel and aluminum engine blocks and copper radiators, and you'll rarely see oxidation or corrosion in an automobile cooling system, and the same concept goes for PC watercooling... but instead of 50/50 water/antifreeze mix in cars, only a 90/10 or even 95/5 water/antifreeze mixture is required for PC water cooling systems