I have about 2.5-3 in. of sand on the bottom. It is not crushed coral ,however, it is coral that has been broken down to sand size. I think it is argonite and I don't know the size of the grain. It is not as fine as the Southdown sand that you hear about, but just a little bit larger grain so that its not flying around in the tank when the power head hits it.
I do use a sump with a skimmer (Aqua C EV 180) and I have a 5 gl refugium with macroalge.
I've only had this tank since April. Its and existing tank that I bought from someone outside of Dallas. After moving it I did have some problems and still do. I made the mistake of keeping the original sand in the tank instead of starting over with new sand. The sand bed was 2 yrs old and had lots of crap that got stirred up into it during the move. My phosphates still are off the chart and the nitrates are very high, however, everything seems to be thriving extrememly well despite the paramaters. I had been getting a little green algae on the sand and glass as well as some red slime. Not real bad but enough to be irritating. It was suggested that I add a couple of power heads. I was using none at the time as the previous owner also did not use them. The return seemed to provide more than enough current. But I guess with my having my rocks set up differently the flow of water was not good. I added the two powerheads and the green algae has not been a problem since the water flow has been increased. When doing some water changes I would gently take off the top layer of sand were the algae was. It was only in a few spots. I've been doing weekly water changes to get my numbers down, sometimes twice a week. The last few weeks I've been doing even larger water changes, about 40 gls. Still hasn't made a difference.
My tank is in front of an air return vent and the vent and some other things are starting to rust and I'm having a few problems with my airconditioner so I will be breaking down the tank and move it on down the wall. It won't look as good in the room, it was in a perfect spot, but maybe it will save my airconditioner. Since I'll be breaking it down I will go ahead and change out the sand with new sand and keep some of the old to seed the new. I'll be looking forward to changing out the water monthly instead of weekly. I've never had a tank before so all of this has been pretty discouraging to me. But since everything is thriving I'm not going to worry about it anymore.
From everything that I hear don't use crushed coral or you'll be sorry. Many are now going to bare bottom but I really like the look of the sand but will keep it to only about 2-3 inches.
Sorry, I may have more than answered your question.
Cindy