betta & tetra...strange behavior; angry or sick?
Four days ago I got 3 bloodfin tetra as tankmates for my betta, at the suggestion of the store worker.
When I first got my betta he seemed more aggressive than the last betta I had--he used to flare at ANYTHING I accidentally placed near the tank, even his food when I dropped it in. Over the past few months, I figured he got more "comfortable" because he rarely flares anymore, generally seems happy. Normally he is a very active fish.
The first 2 days of having the tetras with the betta everything seemed okay, maybe I am humanizing my betta too much but I am not sure he is happy with them. They seemed to avoid him and vice versa, although yesterday I noticed my betta has been hiding in the corners, trying to bury himself beneath things, hiding in plants. I know the "hiding" is normal to an extent, but I have actually found him lodged stuck under things in the past few hours and I have to UN-lodge him. This morning I noticed one of the tetra was missing, and found him stuck behind the cave sort of bloodied under his gills and missing an eye, dead. It seemed fine/active/happy last night so I am assuming my betta attacked it and it hid behind the cave where i found it.
Since then my betta's weirdness has increased, but the other 2 tetra still seem fine. he has been at the bottom of the tank most of the day.
what are the chances he is sick vs. him being unhappy with tankmates, or even maybe him having caught something from the tetra? is betta with bloodfin tetra not a good match? I've read of some "fin nipping" but I haven't seen that, since they really do not interact and my betta seems to chase them away if they come near (which they really don't)
I HAVE to do a water change since that one fish was dead and the tetra food is dirtying up the water super fast. I am about to go get betta revive...but I found it curious there were such drastic changes in a two day span, so I'm not sure whether he is very sick, or very stressed, or very sick because he is very stressed..etc. Any suggestions would be appreciated....let me know if i am making some OBVIOUS mistake please.
and tank info if needed: 10 gallon tank, filtered, heated at about 77-78 degrees. 1 real, 1 fake plant.