Is this Diatoms/Brown Algae OR Iron+Manganese problem?

Hi all,

I'm struggling with an issue in my tank. I have this issue since 2nd month after setting up the tank. It is 5th month now. The issue is brown film like deposits on aquarium glass.

tank size : 48"L x 35"H x 40"W
lighting : 2 x 28W T5 fluorescent tube 6500K
It is planted moderate.
Wet/Dry sump filter - no activated carbon or chemical filtration media (for phosphates or silicates).


Please refer to the pictures of the side and back aquarium panels.



The top portion of the glass is very clear and the bottom portion is covered with a brown film. This brown deposit extends the entire length of the glass panels. The pictures indicate the clear boundary between the two regions.

The region where glass is clear receives less light. The region that has brown deposits receives more intense light - due to the reflector arrangement in the lighting.

I clean the front glass panel very often, but this deposits will start to form again within few days. it will begin to form as small patches and eventually spread across. I stopped cleaning the side and back glasses.

I originally thought that these are Diatoms. Brown deposits were seen on leaves of the plants too.

Now, I'm suspecting that it could also be because of high Iron/Manganese content in the well water that I use in aquarium. I cant afford iron/manganese test kit right now. Also, the water is hard and could have high silicates/phosphates. But, sorry, i cant afford so many test kits currently.

Please let me know if anyone had similar experience. Does it look like Diatoms or Iron/Manganese deposits ?

Also, in the 2nd picture, please note the brown deposit on the diy overflow pvc parts that are submerged inside the tank. Here again, the top portion is white and the bottom portion is dark brown.

Thanks in advance.