You’re gonna love this stretch. But I’ve had a spawning pair of mandarins (still do) for 3+ years. As my lights ramp down in the evening, this is the time that the female and male together swim to the top of the tank. She hits the surface expels her eggs an makes a quick u-turn for the bottom of the tank. The male quickly follows after he’s fertilize the eggs. So here’s what I don’t know , your female Mandarin may be exhibiting a sort of readiness to breed? This may be the case in the wild where they actually not just break the water but also come out of it. Mandarins spawn in late sunset or when lights are ramping down because most fish are already finding their places to rest for the night. Also the Mandarin eggs float on the water’s surface. Not great odds of survival when you have a large tank with an overflow.