I also suggest you drill the tank for a sump. My reasoning is to hide the equipment, but also because in-sump skimmers tend to be cheaper and better quality than hang-on. You can get drill bits from Princess Auto that actually work quite well and are cheap. Practice on an old 10 gallon tank. Use some sort of guide like use a hole saw in a piece of plywood, then tape it to the tank. The glass on a 10 is really thin and they break very easily. So if you can drill a 10 you can easily drill a 125. My personal rule of thumb is whatever the size of the hole, I don't go any closer than that to the edges. If you decide to bottom drill, check the tank manufacturer, some temper the bottoms and you can't drill tempered glass (it will shatter). Look online for how-to's for drilling and installing and overflow. I recommend a Herbie style drain, and because of the length of your tank I would suggest at least a partial coast to coast overflow because Herbie style drains are better used on single overflows, not dual.

I am also not a fan of using canister filters on reef tanks, especially large reef tanks. If you do use it though, do as "madreefer" suggests regarding what to put in it. Expect to have to clean it once a week at least or you are likely to end up fighting high nitrate issues (read: algae). For what the FX5 can do (polyster media, carbon, and GFO) you can easily achieve in a sump where maintenance is much, much easier.

^ +1 what she said. Plus, I will add that, if you have the means to do a sump at this early stage (ie, you have the space, time and money), then I don't think there is a reason NOT to use a sump. If you can do a sump now but decide not to, you will probably regret it later once you get going and start wishing you had a better place to store your heater/skimmer/reactor/ATO, etc. At that stage in the game it will becoming annoyingly difficult to install a sump.

You should be able to sell that filter for at least $100 which will get you a decent sump.

I currently use a FX5 on a 90 gallon reef and its a major pain in the a**! I wouldn't advise anyone to go that route. Put a sump in now, as you will regret it later. They are a great filter for a freshwater tank but not really made with saltwater in mind. Listen to everyone and save yourself the time and headache. Just my 2 cents...

ok going with a sump. my light is a 250 double ended HQI metal Halide?
by Coralife. I do not know the exact depth of the tank but prob around 30 inches or so.

You would need one of those for every 2 feet of tank, so another 2, but again that depending on what you want to do. You may want to sell that and get a full length light fixture, there are nice ones with built in timers and things. And you can also get combined t5 and MH fixtures. If your not gonna be running high demand corals you may want to go with a all t5 setup (less heat) or even an LED setup. Unfortunately I dont know much about LED setups and because they are fairly new there is alot of misinformation out there. But if you can find a good setup they have very low power consumption and almost zero heat.

the light i have has T% and Halide combined right now.

Does it go across your whole tank? how many bulbs total?

Is your tank 4 feet long? Is it a 110 x-high... 48 x 18 x 30"??? You will need to know what size and dimensions your tank is for figuring out how much carbon, GFO, etc as well as fish suggestions.

So your suggesting that he put 1000 Watts of metal halide over this tank? Plus the T5's which I doubt they are. Most likely PC's if it's Coralife. I would assume this tank is 48" long but even a 36" will do. Coralife doesnt build anything shorter if it's 2 250W combined with supplemental lighting and moon rays. 30" does sound a little deep for a 125G
What he has is fine for this tank if he is going to use this Coralife fixture. If he adds another 2 like you suggest he will need a pretty good chiller.