These 2 here were my pair of JD's that I had up until a year ago when their tank sprung a leak and I had to re-home em. The male never had a name while the female was named (Alduin the 2nd) the original Alduin was over 10years old when he died. He was also the biggest JD I've ever seen at 14in, he was also surprisingly the calmest and most chill JD I ever had either.
The 2 in the pics tho were my pair I had after Alduin passed away. The male was roughly 8in while the female seemed to be stuck at only 6in. Sadly never did get any fry from my 2, I think the main problem was was the female was half blind and due to this she probably didn't feel safe enough to want to breed. Oh well tho, I atleast got a few years of joy out of em before rehoming em to someone else in the end.
P.s. I'd agree with everyone else saying that the 4in mark seems to be the point where males and females start to develop differences.
Granted tho, taking a second look at Alduin in the bottom pic. I'm thinking that was actually a male rather then a female lol. So I guess that would also explain why there was never any fry.