Thin layer film growing on silicone

[ixTCxi]

So I silicone the back chamber my self and I notice this thin layer of film growing on the silicone inside the tank, every time when I clean it, it comes back. What could it be? the tank is filled with water , rocks and sand, cycling.

 

[xxxAngeloxxx]

take a picture if you can to see what your really talking about