Surface oil n brown slime

Question
Hi Karen.
I'm keeping one asian arowana (5inch) for abouut 5month. last week i move the aro to a new tank. Now, i got an oily water surface n a brown slime at my tank silicone (just the silicone).
The detail:
old tank 0.8mX0.45mX0.45m +- 30gal (hv no problem)
new tank 1.5mX0.5mX0.5M  +- 80gal
i use ceramic ring from old tank
light 2X14w household flourcent on from 7am to 10pm
filter DIY dry wet filter, wool->bio ball->ceramic ring->bio sponge. filter had brown slime too even  got no light from lamp.
tank got a fairy low indirect sunlight
WC weekly, 20%
feed mainly pellet food no live food
no plant
Filter capacity 1000gph
I'm want to know is  the oily surface harmfull?
what is the brown slime?
is there a way to get rid the oil with no chemical?
thank you for your time n answer.

Answer
Hi there,
The oily surface is what's called a "biofilm" that forms a thin sheet on the water surface due to pollutants in the tank and even particles in the air. You can usually remove it by increasing the surface agitation using the filter outlet or an airpump/airstone and doing more frequent water changes.

Brown slime algae is common in new aquariums and may be present for awhile until everything becomes established. It may also be a sign that there are too many pollutants in the environment. Again, water changes at the key.

Your Arowana needs larger water changes to help him grow and be at his healthiest. Try doing them twice a week instead of once and increase the percentage to at least 30-50%

I hope this helps and best of luck!
Karen~