15 Neon Tetras x 1.5" each = 22.5"
6 Golden Tetras x 2" each = 12"
6 Red Eyed Tetras x 2" each = 12"
4 Blue Tetras x 1.5" each = 6"
97 L = about 25.6 Gal (US) or let's just call it 26" of fish to fit your size tank. You have 22.5 + 12 + 12 + 6 = 52.5" of fish. Even if they aren't at their full adult size yet and are only around 1" each, you are still talking about 31" of fish in a tank meant for 26". You are over stocked and the Blue tetras are responding to that it seems.
Take a look at this page and what they say about their behavior. If you are interested in getting more tanks in the future then as they get bigger you will have other options. But even if you don't at this point you might have to do a little more regular water changes and vacuuming to keep the quality of water good enough for that amount of fish. Have you checked your ammonia, nitrite, and nitrate levels lately to be sure everything is in line? Changes in water quality can also lead to misbehavior in fish too, but it sounds just like an over crowding issue in this case.