SFS are soluble fiber supplements, which you're already taking. I think it would be fine to eat insoluble fiber with fish as long as you had a good portion of SF food, like GF bread, potatoes, or rice.
I know it's frustrating to realize that IBS can cause so much pain. It may take a while of following the diet for you to feel a lot better, but most of us noticed some improvement each week. For me, I would say it took a week or two for the diarrhea and gas to resolve and about four months for a significant reduction in pain. Just try to be patient. Actually, I should read my old posts because I'm forgetting my personal IBS-improvement timeline.
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I think it would be fine to eat insoluble fiber with fish as long as you had a good portion of SF food, like GF bread, potatoes, or rice.
Actually, I should read my old posts because I'm forgetting my personal IBS-improvement timeline. 
