Oily Residue on top off water
I am a recently new purchaser of a salt water tank setup, 180 L, Live Rock and one awsome little snowflake eel. i have noticed a oily residue on top of the water, he has been in there for 3 days now. I bought a Lion fish yesterday and he sadly didn't make it through the night i came home this morning and he was floating on the bottom of the tank dead. I read some where about something to do with the eel leaving a residue of oil and it affecting other fish in the water. Can any one help me with this issue, is it common ? and what can i do in the future to stop/control this problem. You don't have to reply or just send me the link to the answer somewhere.