Brown spots on silicone

[RaymondL]

For those that might have clear silicone based tanks: have you ever experienced discoloration or build up of brown 'stuff' on the silicone? I have some weird dark brown spots on some areas of the silicone. This is a 4 week tank that's cycling - no lights, no bioload - just bacteria starter from Frtiz.

I wonder what this could be. I tried to remove with my finger, but it is difficult to remove without pressing hard.

 

[AlyciaMarie Verified Admin]

Do you have pictures?