Contamination Black spots on machined 7075-T73 Aluminum

Hello,
My question is that once I machine 7075-T73 aluminium forgings I am seeing black spots on machined surface.I looked at coolant concentration 15% and PH value around 9.0.I do see the coolant having small amounts of leaked hydraulic and spindle oils.Also the coolant tanks are cleaned every 3 months.I am unable to know what is causing this problem as it is occurring only on a few machines in the FMS cell and not all the machines.I tried to put a cut piece of Aluminum in coolant sample and it turned out to be contaminated.I put the sample in the washing liquid used in the part washer and it looks good as the washer is the common station for the whole FMS cell and that all parts passing through the washer are not affected.When I put the sample on the fixture material there is slight color change on contact area which could be galvanic action.But seeing the black spots on non contacted machined surface is what I am unable to get.What should I add in coolant to see that I don't have to get the parts deburred after machining to remove contamination.
Srinath