chemi pure, purigen, 2 gfo reactors and carbon makes me think there's an issue, like not enough water changes, or trying to fix a nutrient spike. It's a balance act between bio load, and your bacteria. Using media by which it's effectiveness varies by nature as it gets exhausted makes that balancing act more difficult. Long term a lil bit can help, too much doesn't. You put new media in, nutrients go down, bacteria dies back from lack of food, media gets exhausted, nutrients go up, bacteria has to build up again.
The coarser the sand, the more likely you are to have these dirty sand issues. So it's a balance act between how coarse/fine your sand, and how much flow you can put to it.
If your sump has no flow on the bottom, which could allow a gunk build up down there, that could be an issue too. Live rock, rubble or whatever can enable that.
Manderins are cool, but in a small tank, especially one without adequate fuge area, your copepod population gets wiped out and copepods help keep things clean.
Don't know what gph are making it into tank with head pressure figured in, but the gph you posted is way too much. Have seen many tanks settle down when they get into that 4-6 the display gph range.
-put dif types of live copepods into tank every few months
-reduce return pump
-add a very small pump into sump to keep debris from settling in there
-experiment with more or dif flow pattern closer to the bottom of display
-depend less on media and more on water changes
-make sure your top off and water change water is below .04 tds
