For those with rays not eating... get them eating first, then hit them with meds. Most antiparasitic meds will knock them off their appitite due to how the drugs work in killing them.
Food items for weak/skinny rays should be the nice fatty stuff. Salmon, earthworms, clam, frozen krill....
As for how to tube feed:
First mix up your food. Use the same stuff (salmon, worms etc...) I highly suggest mixing in some kind of appitite stimulant (B12 from your local store works. Crush up about 100mg of it per animals you will be force feeding). I also suggest using a bit of a high calorie addition (Nutri Cal, STAT or something similar). Use a blender to make a nice thick cream or paste of food. Get a syringe ready, attach a short length of airline tubing to it. Make sure the tube ends are smooth and not jagged. In the tip of the syringe, put in your dose of panacur. Put the food in behind (on top of the meds to make sure the meds go in first).
Usually it helps to have two people to tube feed. One person to hold the ray, and one to feed it. Preload the airline tubing with food, so you aren't pushing air into the animal. Catch the ray, and hold it inverted out of the water. I usually support the ray with one hand, and hold the tail with the other (it goes without saying, watch those barbs!). Once you have the animal flat and calm, slide the tube into it's mouth. Go slowly, remember the stomach is fairly far down the body cavity. If you go slowly, the ray will usually start to bite/swallow the tube for you. If you feel the back of the stomach, pull back slightly. Just be careful not to force anything. There is the risk of tearing the inside of the stomach. Once you are in the stomach, slowly push the food in (Amount will vary with size of the ray). Once you are done pushing food in, pinch the top of the airline tubing, and slowly pull it out of the rays mouth. Put the ray right side up slowly, and put it back in the water.
I'll work on loading some pics later to show this process better. Hope that helps give an idea of the process.