Clowns need two things: numbers, and space. They swim like beasts for the first couple of years and require a decent amount of swimming space. They will be super active and happy in plentiful numbers, which could easily be a good sized school in your 125g. Filtration-wise I think a sump would probably be your best bet regarding the size of your tank. I have 2 hob filters on my 100g (emperor 400s or whatever they call them now), but am probably going to switch to a sump once I move (purely aesthetics here, not performance-based switch). The hobs are fantastic filters but not so much for 125+g tanks because of the size and how they fit on the tank. It all comes down to personal preference, though. If you can fit them, and like them, then just do hobs
they break the surface of the water for you and provide a decent current for fish like clown loaches. Plus they are easy to clean, and you don't really have to worry about them back-flowing or anything nasty. But make sure you get a decent turnover rate, though, because your loaches are big waste producers once they get large. Don't just get a 125g rated filter and think it's going to work, because you will need more capacity than that to keep your ammonia and nit levels down. I have 160g worth of filtration on my 100g, but do 3-4 60% water changes a month just for my loaches all the same. I have two hand-sized loaches that I'm going to try and find friends for when I move. They drove out/killed a few smaller clowns I tried to add, which does sometimes happen.
They are fantastic and very curious fish, even once they sort of slow down. My main advice here is
do your water changes. Clown loaches do not respond well to old tank syndrome and other bad water situations, but thrive with enough filtration and regular water changes. I do 60% every 7-10 days, and my test kit stay happy, lol. Sorry for the rant. Anyways, regarding layout of the tank...
Driftwood and plants are a must to create a naturally peaceful tank for clown loaches. Not necessary, but they go way far in kicking up the activity and security of your loaches. Plus the plants help the water quality
I would also make plenty of caves. My two loaches alternate between 4-5 different sleeping spots each day, and use them as base camp for hunting snails at night. It also helps a TON with letting them feel safe. Lots of loaches get scared easily, because of their twitchy nature, so plenty of cover on the ground and mid levels of the tank go a long way.
/rant.