If I was going to move a 120gallon tank, I'd get a plastic cooler, like for camping. Put it in the bed of the truck and use the cannister to fill that cooler with all the water it will hold. 20 gallons?
Net the fishies and put them in bags. Plop them in the cooler. That water will act as an insulator for the ride as well as a temp buffer for them.
Drain the rest of the tank. Haul it off.
Don't crack the cannister. Leave it wet and recirc it in a 5 gallon bucket until your ready to move it. Leave it running and pumping in that bucket until you have the truck running in the driveway and ready to drive off. Un-plug it and haul your butt off to the new location.
As soon as you get to the new location, you need to set up the stand or find the location and get the tank set. Fill it with tap water--less the 20 gallons or so in the cooler. Plug in the pumps and the filters and get that water moving. Add a de-chlorinating agent to the tap water. Maybe "overdose" it just a little bit with the de-chlorinating agent.
Now set the fishies--in their bags--in the tank and fill it up the rest of the way with the water from the cooler. Try and get the tap water as close to the current water temp as possible. This is stressful for the fishies and dumping a drastic temp change on top of it all may just kill them--so water temp is crucial.
Let the fishies float around while the tank clears up a bit. This will stabilize them slowly to any minor temp differences. 15--30 minutes +/-
Now open the bag and introduce a few ounces of water from the tank into the bags. If the fishies are in 1 quart of water, I'd introduce about 2 ounces of tank water to them every 15 minutes. Monitor their reaction. Adjust water transfer and timing accordingly. If they looked stressed--then let them sit for a bit. If they are happy--then procede with water transfer from the tank into the bags on 15 minute intervals.
When the bags are full of tank water, they shold be just about acclimated to that. Dump 'em out and pray.
I'd use those fishies to "seed" the tank again for a month or 6 weeks before you invest big $$ in Cichlids. You want to make sure that tank is well seasoned and stable before you drop coin on those expensive Cichlids.
Trade the Silver Dollars in for Cichlids when your ready to stock the tank. If the LFS won't give you credit--just give them away. Better than flushing them--thats just cruel.
I've done it. I had to move a 135 gallon salt tank about 20yrs ago. It was a 2hr ride to college and I had about $1500 worth of fishies in that tank. They all survived, but it was about 10hrs from the time I netted the fish to the time I released them back into the tank. Thats was a tremendous amount of work.
