It's been some years now since I had my Freshwater Tropicals, but I did keep fish for many years. My memory is of how large the gill movements can at times be, particularly with Angel fish! - Also they could open their mouths alarmingly wide! -
(enough to swallow a Neon!) - and at times I'm afraid they did !!!
I agree with what others have said, - that the gill movement is not always obvious with some fish, and varies over time. - ...... But they all have to breathe!
Have you not got a Tropical Fish store you could visit?
Edit! - Jim, Why not put a few fish in that tank you have?