My guess is that the food compacted in the delivery tube. I've found that even a with a watery slurry, the particles will pack together and jam the tube. I've used Super Selco and liquified flake food very well. Be careful about "eyeballing" vitamins - you could easily overdose fat soluble ones if you aren't careful.
Back in my pet store days, I used to just grab up fish to tube feed them, but now I won't do it unless the fish is sedated with MS-222.
Here is an excerpt from my Advanced Marine Aquarium Techniques book that outlines the technique I use: (Copyright 2006 TFH pub.)
To determine exactly why a particular fish isn’t feeding, a bit of analytical work is in order. Anorexia (lack of feeding response) in fish can stem from a variety of causes, and thus has many potential solutions – with only one best solution for each case. In addition, there are some causes of anorexia in captive fish that have no reasonable solution, and therefore are hopeless. In determining why a fish will not eat, you first need to determine if this particular species of fish has a history of not feeding well in captivity.
1) Is this species of fish known to be a poor feeder in captivity? If it is one of the “impossible to feed” species – it may never begin feeding on its own and tube feeding it will only prolong its life, not allow it to thrive and grow.
2) Has this individual fish fed well in captivity in the past? If it has fed before, what has changed to make it stop feeding? Tube feeding should only be considered in this case if rectifying the problem will take more than a week, otherwise it is usually better to simply let the animal fast until you fix the problem and it begins feeding on its own again.
3) If the animal is suffering from a disease, is the prognosis good enough to consider tube feeding it during the treatment?
4) Does the animal require a treatment with a medication that cannot be administered except by tube feeding?
Force-feeding or tube feeding are both terms that describe an attempt to give sustenance to an animal that refuses to accept food on its own accord. How long can a fish go without food and still survive? Some larval fish at critical stages of their development may starve to death in less than a day if the proper food is not made available to them. At the other end of the spectrum are cases where moray eels have gone on self-imposed fasts lasting as long as 8 months, and then spontaneously began to feed again. A very large saddleback butterflyfish refused all food for over 120 days and when it finally began to feed on standard aquarium fare, it did not look any thinner than the day it arrived from the overseas collector. Generally, unless there is some underlying acute disease or water quality problem, otherwise healthy fish can undergo long periods of fasting and still survive – if the cause of their anorexia is eventually resolved.
The food recipe used in tube feeding changes little from one fish species to another. The basic premise is to prepare a food that is liquid enough to easily pass through the feeding syringe and tube, but thick enough to carry a high amount of calories to the animal’s digestive track with the least amount of water. Avoid trying to mix a “super food” chock full of extra minerals, vitamins, fats and protein – stick to a mixture that is similar to the animal’s normal diet. Regular aquarium flake food blended with an appropriate amount of water suffices in most instances. Upon a veterinarian’s recommendation, some experimentation was done using a convalescing pet bird product known as “Critical Care”. Evidently, this product contains a mixture of amino acids, the precursors to proteins, and these are more easily digested by ailing animals. This product was mixed into a slurry and tube-fed over a course of days to some fish with no problems noted.
The amount of food to administer is difficult to judge, but generally, an amount approximately 2 to 4% of the animal’s body weight is sufficient. This should be just enough food to cause a slight distension of the animal’s belly. Too much food will usually result in regurgitation – often nearly the entire amount that was fed.
Tube-feeding equipment varies depending on the size of the fish being fed, but normally consists of a syringe and a flexible plastic tube. Feline urinary catheters or avian tracheal tubes can be used for small fishes. Larger fishes can be fed using airline tubing or horse catheters. Many prefer to use a Luer-lock syringe so the tube is held more securely to the syringe body, but the tubing can also be glued to the syringe with cyanoacrylate glue (Noga 2000). The tubing needs to be rigid enough to pass into the fish’s stomach but soft enough not to injure the lining of the gastrointestinal tract (Noga 2000). It helps with insertion if you first bevel the end of the tube. The tube must also have marks on it at regular intervals so that you can more easily judge the depth of insertion.
The actual tube-feeding technique used is difficult to describe, and a person’s first few attempts are sometimes met with failure regardless of the instruction given. In many cases, the use of an anesthetic such as MS-222 is required to quiet the fish so the tube can be inserted. In some other cases, the fish can be manually restrained for the few seconds it takes to administer the food. It is generally best to first hold the tube alongside the fish’s body and mark the length needed to reach the center of the animal’s stomach. Holding the fish on its back, gently insert the tube into its mouth and try to locate the esophagus. If you are too far to the left or right, the tube will usually emerge from one of the opercula. If you have the tube positioned too high or low, it will often hang up on the tongue or vomer teeth in the roof of the mouth. Once the esophagus is located, gentle pressure is all that is needed to insert the tube down into the stomach. At that point, gently press on the syringe plunger and inject the food into the fish’s stomach. Gently withdraw the feeding tube and return the animal to a recovery tank. Keeping the animal submerged during the procedure and at all times afterwards minimizes torsion on the stomach and will reduce the chance of spontaneous regurgitation. The frequency of administering this procedure is a compromise between getting enough food into the animal while reducing the stress inherent with tube feeding. For example, a small marine angelfish might normally be fed two or three times per day, but tube feeding an animal that often is likely to cause chronic injuries that negate the benefit of the food being given. Furthermore, fish that have just recently been tube-fed are not likely to begin feeding on their own. Therefore, it is best to tube-feed small fish every second or even every third day, and always try to entice the animal to feed on its own just prior to each procedure. Some large predatory fish may only need to be tube-fed once a week or so.
What are the main drawbacks to this procedure? One problem is that until one develops a practiced technique, improperly tube feeding a fish may cause serious damage to the animal’s digestive system, or at the very least, will prove ineffectual at providing any sustenance for the animal. One can describe all the equipment, the food recipes, and when you should or should not attempt this procedure – but your results hinge entirely on how well you are able to carry out the process. This is akin to why some doctors, despite their similar training become surgeons while others prefer general practice. Remember, “Practice make better.” Perhaps the greatest cause of failure with this technique is picking a fish with an “impossible” prognosis: An animal that is dying from an unresolved protozoan disease is not going to be helped by a tube-feeding procedure. A fish that is within days of death from starvation is also not a good candidate for this technique. Knowing which fish to give up on, and which may actually benefit from tube feeding is of paramount importance to the aquarist.
Jay
