It's a girl..!

Today I got confirmed that Midnight my baby single tail Butterfly is a girl !

I didn't know fer sure, because he isn't as round as Braampje is.

But this is why I know!

When you look at the goldfish's vent area, near the anal fin, You'll see that a female goldfish's vent will protrude very slightly beyond her abdomen, appearing as a raised surface beneath her belly. 

 By comparison, a male's vent area will completely flat, and its anal fin will appear thin.

 

Female

 

Male

 Male and female goldfish have slightly different shapes. Male goldfish are usually longer and thinner, with a more torpedo-shaped appearance, when compared to females of the same age and variety. Females, in general, are rounder and thicker, with more flesh around the abdomen.

As you can see, Braampje is a lot more rounder then Boeddha is,

First Midnight was almost the same shape as Boeddha is, but she is getting more and more round.

You can also see that on the first photo on the page of Midnight.

  

When ready to spawn, male goldfish develop small white spots on their gill-shields and pectoral fin, known as breeding tubercles (breeding stars). Not all males exhibit breeding stars, but, when they do appear, they are a certain sign that the fish is male. Tubercles may last for several days or weeks, depending on the male's maturity and biology.

The second day I had Boeddha he also showed some stars on his pectoral fin.

There were also eggs in my tank that day, I didn't saw the Breeding stars after that day anymore though.

I didn't make a photo of it so I searched up this one that looked just like him !

 If possible, look at the underbelly of the goldfish to determine its sex. A male goldfish has a "midline ridge," or a raised line that extends from the back of the pelvic fins to the vent. The ridge is either completely absent or very faint in females. Unfortunately, it can be difficult to look at a goldfish closely enough to examine for signs of the midline ridge.

I don't have a photo of my fish to show you, but I did check it and what I saw is that Boeddha has this line from his pelvic fins to his vent that is a bit set. 

Braampje and Midnight do have that line a bit, but it's just flat on their belly

Male goldfish tend to chase other fish, particularly females, and may nudge the females' rears after catching up to them. If you notice that one fish is almost always the chaser, and that the other is usually the "chasee," odds are good that the more aggressive fish is male and that the victms are female.

As you can see Boeddha is always behind the other two!
 So I think I can gladly say that Midnigt and Braampje are female and Boeddha is a male!