I think you depth is ok. Shallower would be better for cleaning but if the animals are happy your good.
I did a lot of research on sandbeds and sub-straits years ago. A true Dsb is as dr schemek calls it mud. Oolitic sand 4 to 6 deep. Some larger grains higher up.
One of our local reefers posted his tank on the boards here and was flamed for using sand from the beach. When I did the research I found nothing bad about using it except organics and critters. Silica and silicates don't leach from it from my research. By leech your saying rock actual stone is dissolving in a tank. If you look at the op pics it's really not that bad. if it was leaching or dissolving it would real bad.
when we say rock in this hobby we are actually wrong in most cases. its not a rock its a skeleton. your live rocl or sand could be only 20 to 50 years old. its porious and elements are likely easily dissolved.
some folks have massive diatom blooms due to silicates. this in theory could be because the "rock" is quite young from an area with a higher silicate content and is more easily dissolved, and depending on what manmade salt is being used the ph is such the elements are easily pulled out.(btw i have never had a diatom bloom, I have natural seawater, possible correlation there.)
I have some "rock" that (sqeeee) I turned over and saw several round "gems" literally the staghorn branches were so old it had petrified and become gemlike(round purple white center like a geode) and then been encrusted over by other corals. That rock is VERY old and is rock, or stone. thats not likely to dissolve. Its also not too likely to be porous enough for bacterial colonization either but its wicked cool.
Crushed coral and live sand is crushed coral skeleton and is porous allowing lots of places for bacteria to live. So with general beach sand Id treat it more like a bare bottom and make sure there is another place for bacteria to do there job like extra live rock in the sump or sand in the fuge etc.....
With Florida beach sand. Different story. Beautiful white sandy beaches. Lots of stony corals being eaten by massive schools of fish that chew the stony corals to digest the algea and animal matter off of it and then poop it out nice and clean and white. Dont believe me? google it.
So Florida beach sand is much better to use than southern california sand for sure, but still not bad.
So this still applies.
And Id say as its cyano in the pic and not algae, its likely PO bound to the sand fueling its growth, as organics would grow algae more freely. IME.
And Im pretty sure silica doesnt grow algae either.
