Sticky oily deposit on indoor pond liner

Hi,

I have a 1000G indoor pond in my basement, I keep cichlids in it, it has
some rocks as decoration and a very thin layer of gravel, I feed fish Hikari
gold pellets, I have a layer of thick sticky oily deposit on all sides of
pond surface, this thing glues to rubber liner very well, I am sure that it
is not algea, it feels none organic and like very fine cream, the color is
very light red (similar to the color of the pellets), if contacts with
fingers, I have to use soap with get rid of it, so it appears to be some
kind of fat, it has no smell.

It is extremely difficult to clean, I have to drain my pond every other
month, and scrape it off with a scraper. Any idea of what is the cause of
the deposit and how to solve the problem?

Thanks.

DJ