News to me.
You could get a meter and check
"I've read that fish oil supplements can raise your FBS and shouldn't be taken by diabetics."
Tell us where you read this; magazine, book or link? I'd like to see how they conducted their testing and whether or not it was on humans, not lab mice/rats. Always read the nutrition label carefully. Also make sure you're not allergic to seafood products like shellfish which some fish oil contain. Fish oil does not raise blood sugar levels. If it does, I would suspect an allergy to seafood as the cause.
This link is a study on 832 humans with results showing no adverse effects on blood sugar levels
And this pdf file care.diabetesjournals.org/content/21/4/494.full.pdf
Personally, I have been taking fish oil for the past 10-12 years and have had zero adverse effects.
I am pre-diabetic. I was type two diabetes, but have dropped all my levels. I take 1000 mg a day of fish oil. Family has heart problem history. My tryglerides were high when I was diagnosed type II, and now they are in normal range. I have no problems taking it.