I've had my betta for just over a month now. Tonight I noticed as soon as I turned on the aquarium night light my betta got rather aggressive towards my neon tetras. He would break up the school and chase after one but did not flare. I have never noticed this before except when I first introduced the neon tetras to the aquarium just over 2 weeks ago. He has never bothered with them after from my knowledge up until this point.
Sometimes as they mature into adulthood they get more aggressive and and similarly as they age can become more accepting of tankmates.
Sometimes you have no problems for months and then one morning wake up to a massacre. Have had this happen myself and seen many surprised Betta keepers with the same situation. Be aware and beware. Seems also to happen more frequently like that in smallish set ups like 5G and less.
FW & SW
Favorite tank right now - 12G Edge nano fish