Replacing substrate without destroying/tearing down the tank

I set up my tank about 4 years ago, with the help of a guy at a LFS that I THOUGHT knew alot more about the hobby than he ACTUALLY knew. Among other things, he told me to keep my PH high, most of my substrate should be crushed coral. Well, serves me right for listening blindly, I took his advice and did 2/3 crushed coral with a layer of fine sand over it so it looked nice. Now, years later, the tank is going strong, but the cheap sand he set me up with has all blown away, and I'm left with this ugly, rocky crushed coral that now looks dirty and dingy.

A guy I knew who decided to go bare bottom gave me gallons and gallons of NICE Oolitic sand that I'd like to put in my tank, but I know the perils of ripping out a full bottom of sand and all the nasties within. What I'd like to do is trade out the crushed coral, removing it from the tank, and replace it with this super fine grain stuff. Question is, how to do it safely? What I do NOT want to do is take my tank down and start over. I have a very nice reef growing and don't want to lose/crush the corals within.

My substrate is about 2" deep currently, though I put a deep sandbed under the rocks that rose up to about 4 inches. I do not intend to touch this, since it's behind my Live Rock wall and well established. I'm really only concerned with the sand 'in front'.

Any advice?

 

you could try a siphon and put it under the crushed coral or remove small amounts at a time. Idk

 

Replacing one half at a time would be ideal. Give it a few weeks between "replacements" to be sure that your bacteria has enough time to repopulate the new sand. If you remove the entire sand bed at once, you stand a good chance of having a huge cycle due to the removal of so much bacteria at one time.

 

I would use a gravel vacuum and clean the half your gonna change out throughly, make sure you have fresh saltwater mixed up already cause your basically doing a water change.
Then scoop out the crushed coral you just cleaned, a plastic shovel from a kids beach pail works great for this. leave the bottom bare till you do the other side in a week or 2. monitor your water parameters closely for any mini cycles, do not do the other side till any cycle that happens is complete.
You can add the new sand as soon as your done cleaning out all the crushed coral your gonna remove, it wont affect any cycle that might happen due to disturbing the CC.

 

how big of a tank is this? what kind of bioload?

 

I asked my LFS dude about this before, and he told me to siphon out half of the old sand (crushed coral in your case), do a water change, wait a week or two, and then siphon out the rest. Then put the new sand in.

 

I'm not sure that there will be enough surface area for the bacteria to repopulate once you've removed half of the existing substrate. That's why I would recommend replacing half of the existing substrate at a time with the new substrate.