Oh, good! Make sure you acclimate him properly! Now I'm pretty sure he will live---they can regrow whole arms if they lose them. If you get him right over there, he should be all right.
And yes, let's talk about your main tank.
Do you have a downflow? A sump? A return pump?
I'm not familiar with your skimmer. Is it a HOB? [Hang on back} or does it fit in a sump?
First, you need something to take the water from your main tank and send it into a sump. This can be a HOB downflow, or a drilled-in downflow, which is built right into your tank. What sends it back from the sump to your tank is your return pump. It should return the whole contents of the tank---as frequently as gravity drops it out...the HOB downflows operate with a big pair of U-tubes that continually siphon the water out and down. Consult the Equipment forum to get a calculation how much pump you need for your system. In that sump you put: your heater, your skimmer (if it's a submersible!), and you can even tuck a little refugium down there, a 'fuge, as it's often called, a low flow section you light, and grow cheatomorpha (a weed) and copepods, to support a healthy tank and lower the amount of algae that grows above.
If you have a downflow and sump already, you're golden. If you don't---it's possible you can make do---again, something I'd ask about in the Equipment forum. You might be able to run your filter empty of all media, and let the rock and sand do all the work.
The deal is, it can. If you've been running filter media, bacteria has grown up in there that is taking part of the food the sandbed should get, so the sandbed bacteria haven't bred as many offspring as they need to. They can, in a matter of days. But this is why you withdraw the filter media bit at a time, to give more food to the sandbed and let it increase to take all the load. When you have no more filter media and when that sandbed is fully going, you should see 0 nitrate, 0 ammonia, and it should pretty well stay that way unless too much food hits the system for it to absorb.
HTH. Keep asking questions. Pictures help. Get an equipment list, get your readings, and let's see if you can get that system running. Sounds like you've got good equipment, but that nitrate-in-the-filter business has trapped a lot of experienced aquarists, because it's the way systems used to work. The berliner system (live sand/live rock, no filtration) came in about 10, 20 years ago, popularized only since about 2000, and that's what you've got the makings of, the new way. It doesn't have spikes and dips in the system every time you change the media, because there are no media, and all you do is empty the skimmer cup.
