if you use gloss, it will create a mirror effect and will stress certain fish since they will try to fight their reflection. I have had bettas that have done this
I dont think it makes a difference. On my 240 I used flat at first...and then when I did a re-coat sooner than I should have the paint bubbled up and I had to scrape it off in spots and repaint. I dont remember if I used flat first and came back with satin...or if I came back with gloss...no matter...I used 2 different finishes and I could not tell where I had to re-paint at all...nada...zip. And I run 2x400w MH lights over the tank and its just all the same black....