I have a 75G that i have setup for just under a month now. the top of the water has a sheen to it almost like runoff water that has motor oil floating on top. atm, i only have 4 cardinal Tetras and 3 Bristlenose plecos in the tank. with some limestone rocks, plants, and 1 ornament from petsmart. I have a marineland emperor 400 hob filter and a External 5 Stage Canister Filter Pump Fish Tank Aquarium I got off amazon. Is this natural am i doing something wrong? do i need to change something in the aquarium?