Actually, fish oil, is a supplement specifically taken because it improves vascular health. The long chain omega-3 fatty acids have been proven to reduce the degree of inflammation within veins by replacing other omega chains that are more prone to inflammatory processes.
Of course, they also tend to relax the arteries which lowers blood pressure. This easing conceivable could lead to a further expansion of existing damaged veins that make up the hemorrhoids.
As you know, hemorrhoids, being a chronic condition, ebb and flow like the tide, sometimes without reason. It's easy to latch on to one thing, but hard to prove whether the cause is direct or just happenstance. Best thing for your hemorrhoids overall is to learn as much about the condition as possible, learn what options are available an then go from there.