Okay I'll give it a shot.
If you want to make the change over - one method many folks here have used ( including myself ) is to get you a large rubbermaid or plastic container.
A 30 gallon would be ideal for your set up - little larger wouldn't hurt.
You need to map out your plan BEFORE starting to do this. Write it down if that helps.
You'll need the sand first - determine what type and how much and buy it. Some folks rinse it - some don't. I don't rinse sand.
Also you need to decide if you plan to replace any of the old saltwater with new saltwater.
If you are adding some new - it should be mixed up and aerated overnight before you start the swap.
A spare heater a MUST if you are using one now.
A spare powerhead is a MUST.
A thermometer is a MUST.
Some dry towels are handy to have around too.
Siphon out about half your tank water to this container with some vinyl tubing.
Place the heater and a powerhead in this container. Turn them on.
What you're trying to do is get the container up and running like your tank is now.
Temperature and circulation is key.
Start moving the live rock over to this container.
Move you corals over to the container.
Siphon off a little more water if you must to insure all the corals are completely covered with water.
Net your fish and transfer them over now.
Hand pick all your snails and crabs and any other critters.
Siphon off the rest of the tank water to this container. Check temperature and provide good water circulation.
Scoop out the nasty CC and discard in a double plastic garbage bag lined 5 gallon bucket or two.
Discard this stuff - it will stink.
Hose out or in the tub - rinse out the tank.
If needed do a quick wipe down with an algae pad or scrape any hard to remove crap from the glass tank walls.
Rinse very well with freshwater.
Wipe it down dry.
Check on your critters in your Rubbermaid container now and then. They should be fine if temp and water is moving.
Set up your 30 back on your stand.
Now here's the debate.
If you plan to place your rock first then you got to do it now.
If you plan to place your live rock on top of the sandbed, then pour in the sandbed to desired depth ( 4-6 inches is nice )
Place a clean glass bowl in the bottom of the tank on top of the sand.
Start pumping ( or pouring ) some of the container saltwater back into the tank about half way.
It will be very CLOUDY. It's normal. Here's where I like to add new tank water. That way I can get my tank full again, and start getting the temperature back up, and start my filters.
Let the tank run several hours if you want.
It clears the tank somewhat.
When you feel ready, and you've checked and tested the new tankwater closely ....
Transfer the rest of the live rock.
Move your corals over.
Net your fish and add them back to the tank.
If you've added any new saltwater to the 30 - I would acclimate these fish as you do when you buy one.
Round up your snails, hermits and such - acclimate them back to the new tank.
That's about it.
Other's may have different methods - this is what I've done before and did not lose any creatures.
good luck
