Do you know if the tank has ever been medicated? Was it used previously for FW or SW? Is it “reef ready”? Are you taking over an existing setup that has fish/inverts?
Research is key to successful SW fish keeping and as lando mentioned reading all the
saltwater articles on this site and the
articles on liveaquaria.com is an excellent way to get acquainted with all that’s required for this expensive hobby.
SW fish keeping isn’t really that much more complicated then keeping FW or brackish fish. The key thing to understand is that SW fish/inverts come from a very stable environment (the ocean) which has a fairly consistent sg/ph and very low nutrients. They require fairly stable conditions in the aquarium as well to thrive long term hence the reason you want to consider a skimmer which removes DOC therefore keeping your nutrient and no3 levels low. A SW tank can be run successfully without a skimmer (as many have done for years) but require more frequent PWC to keep DOC down. Since SW doesn’t exchange oxygen as well as FW you are also limited in the amount of fish you can keep compared to FW or Brackish. For SW the general rule is 1” of fish per 5 gal for starting out. For FW/Brackish it’s 1” per gal. There is virtually no limit on how many inverts you can keep though within reason. Acclimation is probably one of the biggest differences between FW/SW and drip acclimating them for 3+ hours to adjust to the sg/ph will help considerably.
FW & Brackish fish come from less then perfect water conditions most of the time and more oxygen rich environments which make them easier to keep in less then perfect water. As already mentioned as long as you take it slow ie: doing a fishless cycle, then adding 1 or 2 fish per week and research your fish/invert selections
before purchasing you should do fine. I would also consider a qt tank (55 gal or a couple of 20 gal tanks) so you can qt all new inverts/fish and never have to treat the main tank.
lr and skimmer filtration (Called the
"Berlin Method") doesn’t require the use of any mechanical filtration ie: sump or hob, but does require water movement through the use of ph (aiming for 1200-2400 gph for your 120 gal). It is easy to maintain although it’s costly to initially setup. You can use a 50/50 mix of
base/lr to reduce costs though and it will serve the same purpose but not look as nice as having all lr.
Also unlike FW or Brackish the use of tap water/de-chlorinator is not recommended due to the high levels of minerals & usually no3/po4. A ro/di unit is recommended for removing all of these elements and to make the water as pure as possible.
A lot of people get them from ebay for around $100. Below are the 3 most popular sites:
Filter-Direct-store (seems to be most popular on this site)
The-Aqua-Safe-Pure-Water-Shop
ROfilterDepot
Also
airwaterice.com is a good place if you don’t want to deal with ebay.