yeah its detritus. my tanks over a year old and I do water changes and sift the sand once a week. I just upgraded my return pump a few days ago and that seems to not let anything settle in the sand so hopefully that will work if not fighting conch here I come lol
The best most effective return pump is around 4-6 times the display tank volume. So you should shoot for around 150 gph with the head height, if any, figured into that equation. Not only does it create less heat and noise, but gives the skimmer, media, and macro time to do it's work. When it comes to a return pump more is not better.
As for a circulation pump in the tank.... Should be around 15-25 the display tank volume per hour. Depending on what corals and type sand you have. And the return pump's gph is a part of that equation.
Guessing you're using ro/di water that's known to be good, so we can rule out silicates causing an issue
If your doing maintenance once a week then you should not have any detritus
Makes me think you have a high bio load what do you have in the tank
With the conch i saw mixed information so makes me think your just going to have another thing adding to your bio load
i have a maroon clown fish, algea blenny, spotted mandarin, fire shrimp, 8 hermit crabs, a few snails, and mostly lps corals. i run two reactors with phosban, the other with carbon, tunze 9004 skimmer, chemipure elite, and purigen with my media basket. i have a 360 gph return pump and a good circualtion pump, and i use rodi water. i just want clean sand
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