Do you siphon the sand when you do water changes? Blow off the rocks before a water change?
Could be detritus and uneaten food settling in the low flow areas and fueling whatever it may be. You can always siphon into a filter sock over a bucket and reuse the water if you don't want to do a water change .
Could be detritus and uneaten food settling in the low flow areas and fueling whatever it may be. You can always siphon into a filter sock over a bucket and reuse the water if you don't want to do a water change .