Hurray! Glad you found the problem! Yes, fish oil can cause soft stools in dogs. Some dogs just don't tolerate it well. If he seemed to benefit from it otherwise--for example, it kept his coat soft and shiny--you can try a lower dose. Many of those gel caps can be poked with a pin so you can just squirt out a half-dose of the oil over food. It's easy enough to just stop the oil again altogether if the symptoms repeat.
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