My knowledge about theobromine is bare bones about this subject, as I am no Vet or chemist, but down through the years I had many discussions with my dog's vets so that I've become pretty familiar with the subject.
I also love to do research on subjects I'm interested in so I've read quite a bit about chocolate, lol.
It's not just that the ingredients in chocolate, or the theobromine, upset the digestive tract, as in, let's say, eating something too greasy or spicy, or just plain too much of anything to quickly. The theobromine itself is toxic in large quantities to humans and animals. It's the quantity that makes it toxic, and in humans we get rid of it quickly, whereas animals don't and so theobromine can reach toxic levels in animals rather quickly, depending on the amount of theobromine in the chocolate.
The darker the chocolate, the more theobromines.
It's just that humans metabolize or get rid of it more quickly than some animals do, so we don't have toxic levels build up, whereas dogs and cats and even horses can have toxic levels build up because they metabolize it much more slowly.
How this will play out with reference to your fish remains to be seen, since we don't know how fast the fish can metabolize theobromine, and also,soince we don't know the concentration of theobromine in the chocolate that the fish ate, which I'm thinking probably isn't much compared to other candies with more chocolate. Peanutbutter cups don't have much chocolate on them really.
I guess that's why my sister's dog, Angie, could eat a pound of milk chocolate and be happy as can be with no toxic effects. If it had been the special dark formula chocolate, maybe a small quantitiy could have been deadly. Who knows?
I really hopeful your fish will be OK, (I'm guessing it will be )please keep us updated.
Here is a reference to the theobromine I found on "chemistry.about.com":
Theobromine is mildly diuretic (increases urine production), is a mild stimulant, and relaxes the smooth muscles of the bronchi in the lungs.
In the human body, theobromine levels are halved between 6-10 hours after consumption.
Theobromine has been used as a drug for its diuretic effect, particularly in cases where cardiac failure has resulted in an accumulation of body fluid. It has been administered with digitalis in order to relieve dilatation.
Because of its ability to dilate blood vessels, theobromine also has been used to treat high blood pressure.
Cocoa and chocolate products may be toxic or lethal to dogs and other domestic animals such as horses because these animals metabolize theobromine more slowly than humans. The heart, central nervous system, and kidneys are affected.
Early signs of theobromine poisoning in dogs include nausea and vomiting, restlessness, diarrhea, muscle tremors, and increased urination or incontinence. The treatment at this stage is to induce vomiting. Cardiac arrhythmias and seizures are symptoms of more advanced poisoning.
Different types of chocolate contain different amounts of theobromine. In general, theobromine levels are higher in dark chocolates (approximately 10 g/kg) than in milk chocolates (1-5 g/kg). Higher quality chocolate tends to contain more theobromine than lower quality chocolate. Cocoa beans naturally contain approximately 300-1200 mg/ounce theobromine (note how variable this is!).
