don't fear the flash on your camera! I ended up with some great shots of my boys!
I have one of Rex, his fins are healing/healed from some tears awhile back, so that's what you'll see in the pic.
The one of Spike shows his colors the best that I have ever got on camera! One thing that stood out the most for Spike was his white ventral fins on his dark body.
You'd need bright lighting to deflect the flash a bit. I occasionally have to tamper with my pics to remove the flash.
Here's one of my watonai goldfish and I targeted the gill cover for my references. I had to use auto mode flash as the watonai was quite agile and pics without flash are quite blurry. The flash makes it less perfect but I got what I wanted anyway.