This is what my water tank looks like on the inside. I have been told to add 28% hydrogen peroxide 1 mil per 4 liters to prevent this sludge from building up.
Here is a photo of the brown sludge:

https://photos.app.goo.gl/rDsUtwxzdpkmJzJf6

PS, I would call you but I am in Canada so 6 hours different and long distance.