Sexing Jack Dempsey's (Pic Included)

Hello,

We bought 4 baby JD from the LFS. These 2 have paired off. So we seperated from the other 2. They are digging a massive hole in the gravel, shaking and pecking at the pot. They lip lock occasionally not often. They also do the tail whipping thing.

The JD on the left is 5-5 1/2 inches long.
The JD on the righ is not quite 4 inches.

Can anyone help me?

Thanks
Bonnie

Here is a beter pic of the fins on the not quite 4 inch JD.