Here is a good link:
http://www.soyaqua.org/asaimusbtech/lvhdcagemanual/fishstress.pdf
This explains how stress in fish will greatly decrease their immune system and general tolerances for change. Often times this will eventually lead to death.
Like I said above, the mandarin is going to starve - you need a set-up that has an established copepod population that could take months to over a year to get. Bottle feeding pods will cost you $30-50 every week in order to keep that fish alive. I say take it to the Local Fish Store and save its life.
I would recommend the same with the Panther grouper, unless you are planning to upgrade to a 300+ gallon tank, that fish needs to go to the LFS as well.
PWC = partial water change - you should do a water change weekly, some people do it bi-weekly depending on set-up and stability of aquarium.
There is quite a bit of an upfront cost to make a 120 gallon tank into saltwater. You will need water, salt, heaters, filters, more than likely a protein skimmer, generally Live Rock and a substrate of sand.
An RODI unit in the long run will be the cheapest to make saltwater: There is a sticky on this site that explains:
http://www.aquariumadvice.com/forum...w-much-to-produce-my-own-ro-water-137331.html
You should also read the (2) sticky threads at the top of this forum as well.
Approximate cost of making saltwater by using the RODI water from a Supermarket at $.29 per gallon + if you buy a bucket of Instand Ocean (IO) salt mix you get 160 gallons for $50, or $.32 / gallon.
Therefore you are looking at (.32+.29) x 120 = about $74 to initially fill your tank and an additional $11 / week in water changes at 15% water change ratio.
LR typically $5-7 per pound at LFS, and possibly on craigslist for $3-5. Base rock you can use for 75% of your rock at $2-5 per pound. You will need about 160 lbs of base rock and 50lbs of LR. BR at $3 = $480 + LR @ $6 = $300.
Looking at around $800 for Rock, if you don't want corals the lights won't need to be that expensive. If you do, add another $300-500 for lights.
Sand, you can use pool filter sand from Home Depot, $5 for 50 lbs - you'll want to get I believe about 250 lbs.
You should buy a refractometer to check your own salinity -
Salinity Refractometer 0-10% ATC Aquarium Salt Water | eBay
That's a nice inexpensive one - $30.
If your tank is already plumbed than you are good to go if not, you will need a HOB (hang-on-back) filter. And again, depending on how you stock the tank, you will want a skimmer - good ones are around $250-$400.
I'm not sure if this helps, but its a good start. Carey just did a 125 gallon build, you could check out her's and other's threads to get a good start.