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When things die, even people, they appear to bloat. Fish are no exception. Although you may never know what caused the dealth of your trigger, I did notice the red ring around the gills of the your trigger by the picture. If not ammonia, I would have suggested it may have been Cyanide Poisoning. They have said the effects depending on the dose can take time. Then again how would you really ever know for sure. However here are some symptoms:

Symptoms of Cyanide poisoning can be difficult to detect, but certain clues can be useful. The effects of cyanide ingestion are very similar to the effects of suffocation. This is because cyanide stops the cells of the body from being able to use oxygen, which all cells need to survive. So the same symptoms that you can get from climbing too high a mountain where the air is too thin also occur with cyanide.

General weakness, confusion, bizarre behavior, excessive sleepiness, coma, shortness of breath, headache, dizziness, and seizures all can be seen with cyanide poisoning. Typically, an acute ingestion will have a dramatic, rapid onset immediately affecting the heart and causing sudden collapse. Or it can immediately affect the brain and cause a seizure or coma.

Chronic poisoning (over time) from ingestion or environmental poisoning will have a more gradual onset.
The skin of a cyanide-poisoned person can sometimes be unusually pink or cherry-red because oxygen will stay in the blood and not get into the cells. The person may also be breathing very fast and have either a very fast or very slow heartbeat. Sometimes the person’s breath can smell like bitter almonds, though this can be difficult to detect.