I used XSPC ECX ultra concentrate coolant and distilled water. I'm running a closed loop PC cooler and the combination works great. I figured that the PC coolant is made for this type of application vs automotive coolant, also it's non toxic and biodegradable so you don't have to worry about disposal when you change the coolant. I know you said you already got something, but if you haven't installed it yet you might want to use the PC coolant.
Dan