Substrates,sand in reef aquariums. good or bad?

If you don’t clean it and if its deeper then 1 inch it’s a wast of time cleaning it, so if cleaned it will not reduce nitrates, each time you clean it you will add oxygenated waters to it killing the nitrate reducing bacteria.
If you don’t clean it, as it gets full of inorganic rubbish trapping organic rubbish, the obligate anaerobes take over from nitrate reducing anaerobes, their bi-product is hydrogen sulphide, this will keep your tanks progress back or can be a complete tanks life killer.
So essentially, in tank substrates are for show only, and that is if no real depth and with on going care!

If you have to have a substrate, go with course particles, say 3 to 5 millimetres so they do not move around with the current and no deeper then one inch or it will pollute your tank in time.
Oh and clean/siphon it each month!
Of course if it builds under your rock, you have a problem!

Cleaners of substrates are a fallacy, those that eat from the substrate shit in the substrate, worms that travel through and eat in it, shit in it and create binding substances that make their tunnels, so they bind it up and put waste in it!
Gobies do not clean substrates; they will sive the surface for benthic life, but cleaning it, another fallacy! Oh and gobies turning it over, near no help there, unless they dig a hole and the edges fill with crap, they just skim the surface looking for benthic life.
You see the main problem with substrates is the grit/sand under your rock; it gets dirtier and dirtier as time goes on and never gets cleaned properly!
The longer a substrate is in play the worse your water quality gets, but most of what comes from a substrate harming your hobby, you can not test for!
The last time I did a functional substrate was in 82, the substrate was 70 mill deep with fully functioning under gravel piping, but it was a reverse flow under gravel with serious prefiltering at the pumps so nothing got under it and what happened above, didn’t matter, the flow was up through, not pull down through it, so no mater who was digging above or living in it, that didn’t matter, plus my rock was connected to my rear glass the rest was on a step of near full width of my tank I built in and it was the height of my substrate and it was 4 inches out from my tanks rear wall with just a sprinkle of sand on it and some rock sitting on it, not on the substrate.
Oh I also had a large sump, the amount of life in that tank that lived so well was incredible!