What causes water to be oily and black

[win618]

Hi,
Anyone here can advise me what causes my freshwater tank to be oily and black? Recently my rednose all die at one go and 1 week later all my neon tetra also die at one go.

Thanks.

[felix_fx2]

Hi,

It is called protein if I don't recall wrongly. You have very thick one?

Are you running canister filter or Hang on?

Btw, oil layer might be also caused if you have sprays also. Maybe you want to share with us your setup.

[win618]

Mine is a canister type. It's a mature tank which was set up 6 years ago.