Did you say 1 inch? in a 120g?
First of all, where ever you got the fish from, question their values. 1 inch is a little too small to sell, not stable enough and too easily die from stress. Pretty anything well stress a 1 inch pleco. It's also a lot harder to get them to eat. Especially in a 120 gallon tank that has other tank mates. Believe it or not, it'll be harder for them to find the food.
I'd suggest moving them to a smaller tank, with sponge filters, and feel them smaller food at first, like granules or even flakes. You can also try gel types of food, like Repashys, it's soilent green is for aufwuchs eaters, which actually describes L200's diet better than veggies. It can be left in the tank for longer periods of them for them to munch on, without breaking down too fast.
I normally would suggest leaving live blackworms in the tank 24/7, but at 1 inch, some of the worms will be longer than the plecos, so I am not sure if it'll be ok.
If you don't want to move them into a smaller tank, move them into a breeder box that you can keep in the main tank. Less to worry about in terms of water quality, they'll be protected, and can find their food without worrying about other fish getting to it. Have smaller pieces of driftwood in the breeder box for them to hide too.
1 inch is really way to small for anyone to sell.... that's just not right......
I agree put them in a smaller tank. I'm growing out L-114 and Pterygoplichthys webari in a 20 L. I can moniter them much easier in that tank. Also I'm giving them water changes every few day and they are growing like crazy.