I'm still trying to learn for sure the sex of my fish. I'm almost certain my big fantail is a female and my big common is a male. If I'm understanding correctly a male with chase a female around the tank with his head under her belly when he wants to spawn. Is this correct that this is a male behavior? I have 2 smaller fish that I'm not sure about, a small fantail and a small black moor. The small fantail is chasing the black moor in this fashion but the black moor seems to be really trying to get away from him. Will a male chase another male in this fashion when they want to spawn? Or will they only chase females?

I've personally had males chase after males, so it's possible they're both males. But, I wouldn't rule out the possibility of the black moor being female! Just wait it out and see what happens. You never know!

Are all these fish in the same tank or pond? I have heard that in the absence of a female males will chase males.

Yes they are in the same tub. I looked at the picture that Koko has posted for us and by that I'm pretty sure my big fantail is a female. Just not sure about the others.