Deep sand bed = nightmare down the road....
My personal experience if anyone remembers the green water incident i had followed after the deep sand bed matter....I had about a 3" sand bed and it started to get weird colored deep down and you could see it through the glass at the bottom (like greyish, black) didnt think anything of it until one day my inverts started dying off for no reason, followed by all my corals, well to make a long story short (and thank god my fish made it through) i freaked and took apart the tank to find nothing wrong...well my last resort to clean out the sand and replenish with fresh new sand, wow is all i can say, the smell from the sand and the way it looked when i got down deep into it,
....Pure disgust is all i can say....will never do a bed deeper than 1.5" ever again!!!
It crashed my entire tank and from what i understand it was hydrogen sulfite/sulfide buildup...and you can test it by smelling the sand, if it smells like rotten eggs you got yourself a problem!!
...and it will eventually create a major problem....Well after that it took about 2 months of trial and error and oh yes green water followed which is rare to us in the marine world, to finally go away.....i can happily say now everything is doing great!! but dam that deep sand bed...Live and you learn i guess....