Approx. 6 weeks ago, I bought a 20 years old Serissa. BRBR
Since 1 week some leaves become completely yellow and fall off.
Some leaves - up to now less than 10 - get black tips. BRBR
Water: Every 2nd day as much as it drips out of the pan a little bit.
BRBR
Once a week Floragard Bonsai fertilizer BRBR
Position: Very bright with no direct sun, BRBR

Your tree is getting too much water & not enough sun or air. It should be
outdoors for the summer in part sun. Leave it out until temperatures go well
below freezing, probably November. Don't water it until the soil starts to dry
out. Don't give it any more fertilizer until it recovers.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)