Red/maroon type coating on rocks ?

[Hilltopreef90]

After I treated my tank for flatworms I lost a few fish and probably several snails and other inverts
I’m noticing a reddish coating on my rocks like when you first start a tank, my tank is a couple years old so I’m wondering where and why I’m getting this type of algae or whatever it is
What’s the best way to get rid of it ?

 

[Tired]

Sounds like cyanobacteria. Check that the water you're using for water changes is pure (no TDS, no chance of dissolved organics or whatnot), and test your nutrients. If you're using clean water, have enough of a cleanup crew, and maintain at least 0.03ppm phosphates and 5-10ppm nitrates, the cyano should fade out. It might be taking advantage of a lack of biodiversity due to things being poisoned away.