@Jay Hemdal any thoughts please
Goiter (enlargement of the thyroid) can cause a fish to have difficult swallowing food, so can other tumors in that area. Can you try to get a look from the front, down its throat while it is trying to swallow? Look for a white or tan colored mass at the bottom on the throat behind the gills.
Here is some text I wrote about that:
Goiter
Goiter, also called thyroid hyperplasia, develops in fishes when the thyroid tissue begins to grow outside of its normal location underneath the gills of the fish, forming a bulging protrusion.
Cause
Goiter can be caused by a lack of iodine in the food or water, or by substances called goitrogens, which are toxic and cause goiters to form in fish exposed to them.
With sharks and rays, high nitrate levels (greater than 30 mg/l NO3-N) inhibit the ability of the thyroid gland to utilize available iodine. This, in turn, disrupts the thyroid gland, causing a goiter in those fish.
Treatment
Potassium iodide additions to the water will rarely resolve goiter in fishes. Aquarists have also tried adding elemental iodine (Lugol’s solution) as well as L-thyroxine to the water, but this rarely cures the problem either.
Administering oral doses of potassium iodide (KI) directly in the animal’s diet is the best way to reduce goiter in fishes. The trouble is that the dose is very small: 2.5 mg of KI per pound of animal weight, placed into a favorite food item. Potassium iodide is very bitter, so it must be placed inside an empty gelatin capsule, or the fish may taste it and reject the food.
This treatment may need to be repeated every 18 months or whenever symptoms of goiter return. Fish that have had a large goiter for a long period may be permanently affected. Also, goiters in such fish can become malignant, generally transforming into a carcinoma, which is untreatable.
