No. No, no, no,no.

Azrael has been basically sexless up to now. It's pretty hard to tell.  Mephistopheles (named by larians) seldom attacks the other fish, but  just looms at them meaningfully.

One of the few ways you can tell the sex of an angel fish is by the appearance of its tiny ovipositor (egg tube) which emerges on sexual maturity. Male ovipositors are slightly pointier, females more eraser shaped. Azrael's ovipositor has suddenly appeared in the last 24 hours.  So all of a moment, we learn that Azrael is not only a boy, but a boy who's ready.

Mephisto's tube is not out yet, so I have no idea... all the other completely unreliable signs indicated young masculinity.
Good options: two boys become buddies/two girls take up and do the whole nesting phantom pregnancy thing. Both happen a lot, both are plausible, both include a bit of roughhousing between the fish.

Bad option. The Thing I Think They Are Preparing For.

No,no,no,no,no,no,no.