the more important issue here is the fact that you have far too many fish that grow large, in that tank. a 75 is not big enough for two mature oscars which grow to 13-14 inches, let alone an eel, a clown knife and a blood parrot cichlid and the rest of your crew. if your "eel" is a ropefish (Erpetoichthys calabaricus) it grows to 3 feet.
unless you're planning an upgrade to about 300 gallons, you'll soon be looking for new homes for some of your fish or they're going to die. fish maintained in tanks which are too small or in conditions which are too crowded, develop severe stress syndrome. these fish do not exhibit proper color, do not develop proper fin form, do not exhibit proper metabolic development, do not exhibit normal behaviour, do not develop proper musculature, do not develop properly functioning organ systems and as a result, slowly lose their inherant resistance to disease. this results in a significantly shortened lifespan and along the way, lots of diseases for which the poorly conditioned fish is a good target.