The tube needs to go in at least an inch more. Spray the tube with Pam cooking spray to help ease it through the esophageal sphincter. Push through the resistance, You will then feel the resistance ease. I would also go with a smaller diameter syringe if possible. The greater the diameter, the increased force it takes to plunge. The high force can be damaging, especially if there is a clog and it breaks free. A 1 or 3cc would be better. The shark could probably handle 1.5 cc. Dehydration can occur with starvation and sometimes I will add Gatorade or pedialyte to the gruel. You should see no food come out of its gills if placed correctly. Good luck. Stat, a liquid to feed newborn sheep, is a milky, high protein and fat diet that I have used too.