have you tried making your own food? i did it with mine to get them feeding. Really simple to do, grab some of the food they're currently eating and stick it in a blender, mix in some pellets and some agar agar power and it should turn to a jelly texture. Put this in some ice cube trays and pop it in the freezer. Works well for weaning any fish of live food etc.
It'll make a mess putting things like garden worms in the blender and obviously you'll have to make sure the mrs doesnt find out what you've been using it for lol.
As for the tank size you'll need to do a fair bit of upgrading, the problem you'll come against if you plan to sell when they get bigger is that not many people will take them. Have a look into Marbled motoros as they generally don't get as big, still need a big tank though. Better still go with Scobina or Hystrix (these crop up from time to time) as mentioned previously my female marble motoro was pushing it in my tank which is 30" wide so for normal motoro you'll need much bigger.