Attach a reducer fitting to the end of a hose and attach the nickel-copper tubing to the other side of the fitting.
Add citric acid according to the citric acid manufacturer's directions to a water tank or large plastic container.
Pump or pour the citric acid-laced water through the nickel-copper tubing to flush impurities from inside the tube.
Run plain water through the nickel-copper tubing to flush out citric acid residue.
Mix 1 tbsp. of a mild, grease-fighting detergent into a bucket of water. Dip a rag into the bucket and wring out excess soapy water. Wipe the nickel-copper tubing with the rag to remove dirt, oils and grease.
Clean the ends that will join with other fittings, reducers or pipe connections with an emery cloth. Wrap the emery cloth around the end of the tubing around twist it around the tubing to remove remaining dirt and smooth the metal.
Insert a pipe reamer into the end of the tubing and twist it around the inside to clean away burrs. File burrs away from the ends of the tubing with a metal file.