Sand/substrates in marine aquariums, why you should not do it!!!

In the short term, yes the sand or rubble or what ever looks cool and attracts aerobic on and with in anaerobic bacteria.
But quite quickly at times velvet and white spot sets up shop in and on your substrate, you see they need that substrate to help increase their numbers in varied ways and also if that sand or rubble is abundant with silica, that helps with diatom out breaks.
With in 6 months to 4 years all substrates become a serious to semi nitrate factory, in that it gets filthy and the valuable nitrate reducing anaerobes get taken over by obligate anaerobes that manufacture hydrogen sulphide.
This screws with your PH and if you disturb a well matured substrate you may end up with a complete tank life wipe out!
Your aquarium is not the ocean, though the shops like it when you think it is, they make more money that away.
You can not have sand like the ocean can with out a lot of support like extra tech gear or hard work on you hobby.
The ocean is of say 1 percent reefs making naturally occurring pollution being waste and toxins from reef life, the other 99 percent of the ocean is open ocean filled with phytoplankton that consumes and subsequently purifies the ocean of all the left over crap once sponges and more get their bit that was made on those reefs.
Parts per million hobbyists!!!! that is our hobby!!!!
In your tank the parts per million in your aquarium of crap is always high no matter what you do, in the vast ocean there is near nil parts per million, due to such a huge expanse of water.