Most people that you ask will say "Dude, pull out the crushed coral".
If you do decide to keep the large particle substrate the just be sure to vacuum the detrious just like you would vacuum the gravel in a freshwater tank. Large particle substrate allows fish food and fish poo to get trapped and decay.
If you choose to go with a sand bed then you should get a minimum of 5 years before you have to change it, most likely longer. Changing the sandbed every 6 months would disrupt the anaerobic bacterial colonies which is the main reason for having a deep sand bed. I would say that you need at least 3-6 months before your sand bed becomes mature, biologically speaking.
Algae will still grow on a sand bed especially diatoms during the first 6 months. After that with regular maintenance including water changes to reduce nutrient buildup and adequate flow your sand bed should be algae free for the most part.
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