Pebbles And Slate With Brownish Spots

[IAmATeaf]

In around 2 weeks some not all of my slate substrate and white pebbles start to get covered in brownish spots, eventually the entire piece of slate chip (around 2-3cm long) or pebble (around 4cm long) will have a layer of brown. But not all of the substrate or even pebbles, just some. I normally take out the individual pieces, give then a scrub with a toothbrush in the tank water during a water change and put them back.

What I'm wondering is why only some? Is this normal as I personally don't like look of film covered slate/pebbles.

What if I had gravel? Do people here with gravel need to scoop sections out to give them a scrub?

 

[waterdrop]

Its extremely easy to get brown algae (diatoms) in tanks where you're not dosing CO2. Sometimes a fairly good floating plant cover can filter the light the right amount to help cut down on algae. This may be easier than trying to find the right amount of light via lower hours. Good high flow rate from the filter and good circulation in general should help (the reason is that circulation helps eliminate pockets of elevated ammonia that is too small for a test to pick up but is enough to work with the light to trigger algae. More plants rather than less tends to help too.

~~waterdrop~~