It is always the question, when is it safe for fish? It is really not that hard, first check that it is really not a water soluble material that feels like oil. For instance some liquid detergents or shampoos look and feel like oil. If the material foams with water, it is probably a soap or detergent. If it is oil, just add dish washing liquid you would use to clean greasy dishes or pots. Get it clean and rinse and dry and the check to see if you can see any residual material. If so, repeat the cleaning. If not, put in some water and if you get a foam, you need to rinse more. If not, you are ready for fish.
If the fish are expensive or rare or you could not live without, get an inexpensive fish and let him live in the vessel for a day to be sure there is no residual materials that could affect your cherished fish. ANd if you have trouble with that, try using daphnia. Daphnia are used worldwide to check water quality. Put in a bunch of daphnia and if most survive, all is well. If all die, you have a problem. Of course, the water used should be from an existing aquarium that supports live fish.
