[Fat Tony]

The darks spots are from sulphur dioxide... not a good thing. This is common with the use of undergravel filters or plenum based systems. Whatever you do, do not disturb the areas as the release of the gas can result in death of alot of your fish if the release is significant.

 

[Fat Tony]

Nassarius Snails are decent sandbed stirrers

 

[Fat Tony]

You have more than enough flow for the tank for sure.... as far as the detritus buildup in the corners... thats a good place for, it makes it easier to siphon it out.