In my 120 I am running a Tunze 9410 dc.
It has been a great skimmer for 21 months now.
Now that the tanks 14 fish have grown considerably, along with all corals, the skimmer is not keeping up.
It is rated for a 315 but Tunze says to subtract 70% for a heavy sps load. That would be 94 gallons.
I only run a skimmer and filter sock for export.
That said I have a 9430 dc on its way and the 9410 dc will go into my 45 frag system as the main form of export.
Yes I know the 9430 dc is slightly oversize, lol.
It is the next size up in the Tunze line that runs a dc pump.
I looked at several other makes but I like Tunze products, warranty, and Roger is great!
The 9430 dc is rated for a 925 gallon system.
Subtract 70% for sps and you get 277 gallons.
I feed heavy at 8-10 cubes of volume a day.
Once the 9430 is online I will setup and auto feeder with pellets.
So for your 120 take a look at the 9410 dc.
I chose to go with another Tunze for a few reasons.
I setup my 9410 from day one.
I have not adjusted it since day one.
Work in the tank.
Glue corals to rocks.
Feed the fish, and I feed heavy.
You never have to mess with this skimmer.
I never turn it off.
Just lift off the cup, clean it, and put it back.
I have removed it several times for a complete cleaning and put it back in and it just plain skims.
I have ran many skimmers in my day.
This is the only one that is truely set and forget, except you need to clean the cup, lol.
Here is 2 days worth of skim from the 120's 9410 dc.