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I should mention that their tank has had some serious water quality problems* that also almost killed one of them so maybe not?..
Maybe my efforts in clearing up this problem with daily water changes is triggering them to want to breed?. Or maybe having the fanciest most expensive filter I've ever run on that tank is doing the trick with the increased water flow?.

Twice now I've seen two of them pressing against each other side by side quivering. I'm going to be excited if this is breeding behavior. If not, then I wonder what they are up to.

* came from installing a new filter and underestimating the bio filter issues that resulted in ammonia raising to unsafe levels. I was at my syno tank nearly 24 hrs saving a syno that at one point lay upside down on the bottom gasping. I do not save many fish in this state, so I'm glad it was a success even though I'm afraid this syno has some longer term issues as well that I hope will heal with time.
I've added new filters on before to tanks without having this issue so I guess it was the size of the new filter that did it.

I have heard that bad water is a spawning trigger, so possibly.

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Mats

The side by side quivering could be a sign of aggression or an early pre courting sign - it depends on exactly how it occurs.

Could you describe it in as much detail as possible?
Foe example are they both facing the same direction? are the pectorals splayed with them sashaying from side to side? Does one look like it is trying to curve it's body round the other fish?

Have you looked at the genital papilae? What size are they? Is it possible to get a pic or video even?

Yeah they were both facing the same direction.
No attempts to curve over each other.

I guess it's most likely an aggression display then.

Maccus wrote:Yeah they were both facing the same direction.
No attempts to curve over each other.

I guess it's most likely an aggression display then.

It is most probably an act of aggression, but without someone with some experience actually seeing it, you can't rule out other options... we'll have to see if anything develops