Ok. I'll give my opinion on what I would do if I were going to attempt the switch. No guarantees and I would definitely seek additional opinions. This is strictly my opinion and I could be mistaken on a few items.
Number one: The best possible solution would be to find someone who could keep your fish for you temporarily until you can add them back to your tank. That may or may not be possible so we will work on the assumption that you can't.
One thing I'm confused about is what you currently have for filtration. Is it the small amiricle wet/dry? If so, this will help.
The one advantage I had was I was not switching tanks and I was able to do this in three parts, sand bed, live rock and then bio-ball removal. As far as having a FOWLR without bioballs, it is definitely possible. You are switching tanks so it will be a little more difficult.
The main thing we want to keep in mind is too keep as much of the biological fitration alive as possible when moving into the new tank. The more we can save, the more ammonia we can convert.
If I'm right about the existing wet/dry, you currently have bacteria in three locations that we can bring over - the wet/dry, your live rock and your sandbed. I would proceed as follows:
1. Have plenty of make up water on hand in case you get ammonia spikes after the conversion.
2. Place the new sand in the bottom of your new 75 and spread it out. I don't know if you want to have a support structure for your rock or if you are just going to place it on top of the sand so decide before you start.
3. Fill your tank about 1/3 full with new RO or RO/DI water matching the salinity and temp of your existing tank as close as possible. If you put some plastic over your sand, it will float up as you add the water and disturb the sand less.
4. Shut down your old tank and move the rock and existing fish to rubbermaid tubs using your existing tank water. You need to put them here because you have to switch your sand over. If you choose to use all new sand, you can just move it directly to your new tank when you get your sandbed setup. You will need to keep in mind that the longer your wet/dry is off, the more bacteria on the bio-balls will die off so you want to complete this start to finish once you begin.
5. Now carefully remove your old sand and place on top of your new sand. If your sand is alive, this will seed your new sand with critters and bacteria. Be very careful if you have bristleworms in your sandbed as they sting. If you are going to use all new sand, use a little of your old sand to help seed it.
6. When you get all the sand added, you will probably have a slight sand storm in your tank. Go ahead and add your live rock the best you can with what visibility you have. Plan on redoing it after the sand settles in a day or so. I would not add any new live rock at this time unless you are absolutely sure it is 100% cured. No need in adding additional ammonia at this point.
7. When you get all your rock in the tank, finish filling up the tank. I would use about 1/2 of the water from the old tank with new makeup water. Install your (hopefully) existing wet/dry and additional equipement needed to get it powered up and then add your fish.
8. At this point we have a new 75 gallon with a 4" DSP, 80 lbs of live rock and our fish which we probably can't see any of at this time. You also have the wet/dry working, skimmer and a return pump.
The important part of this is the existing wet/dry. It will continue to provide biological filtration for you along with your live rock. The sandbed is pretty much going to be useless for a few weeks until it settles in and starts developing the bacteria that will help you eliminate nitrates which is what makes a DSB rock.
Do not feed your tank for a couple of days. The fish will be fine and they are going to be a little stressed anyway after the move. The addition of food just adds more waste which we do not need right now.
Monitor your ammonia a couple of times a day to make sure you don't get a spike. If you do, keep a close eye on it as it may be very small and not require any attention. If it is large, you may have to do a water change or use some kind of ammolock product. I've never used them and really can't tell you anything about them. They may not be good to use so others may be more helpful with them.
If all goes perfect, you should settle right in with stable water readings. If ammo and nitrites stay or get down to zero after a week or so, start removing a few bio-balls every week. I removed around 12 or so at a time. Keep an eye on your ammo again. The bacteria you remove will have to be replaced and the only thing to replace it will be the live rock and sandbed. Keep this up until all the bio-balls are gone. If you get to a point where you remove the bio-balls and the ammo becomes unstable, you may have to add some more live rock for additional filtration or remove some fish. Keep in mind what I mentioned before. The more porous the live rock, the better filtration it provides. 1 lb of fiji will filter about as much as 2 lbs of Fl. aquacultured rock. Fiji is not the only porous rock, but the one I am most familiar with.
Once you get rid of the bio-balls, don't waste your money on a new wet/dry just to use as a sump. 29 Gallon aquariums are very popular to use as sumps and much cheaper.
Hopefully some others with more experience will chime in and add to what I've recommended and correct me or offer better ways to do some things.
Good luck and I'll check back if you have any questions, but it is usually late at night before I can get on. Hopefully I did not forget anything.
Tangman
