Alright. So I purchased two baby Oscars (red and tiger) from Petco a little bit over a week ago. They live in a 40-gallon tank ( i will upgrade later, don't worry) with two baby sun catfish and two baby common plecos (which I added today). I apologize in advance for the amount of reading you guys will have to do, but I don't know what info I need to include and what I don't, so imma just add a lot of info.
My tank is not fully cycled, but I don't think that's why my Tiger Oscar died. Before two days ago, I only had my two Oscars and the catfish in the tank. Then, two days ago, I added the second baby sun cat to keep the first one company. as soon as I released the cat into the tank, the two Oscars began to harass it. so I removed them and put them into a quarantine tank. after about 40 minutes, I put the Oscars back into their original tank. They didn't harass the new cat anymore. but they got really moody. like really sad; they sulked. so I did some research.
I read here on MFK that Oscars can get moody/sad when they're moved from tanks, tank decor is rearranged, etc. so I figured they'd be okay the next day. yesterday I woke up, and my red oscar was swimming throughout the tank, but the tiger oscar wasn't. I figured to give him some time. later on, I tried to feed my Oscars. (note: I have been having feeding problems, finding the correct food they like. I figured out that they liked sinking pellets, but I had floating pellets. so I bought some when I purchased my second cat). And the Red Oscar finally ate. The Tiger Oscar didn't eat cause he was hiding this whole time.
This went on the whole day. He barely moved all day. I figured he would be back to normal the next day (today). When I went to bed, I turned off the lights and turned on the moonlights. After some time, the Tiger Oscar eventually came out and moved about just a bit. but not much. This morning, I woke up to my Tiger Oscar laying on the tank floor. Dead. So I went and bought a replacement Oscar and the two plecos.
So my question is, why did it die? My other fish, Red Oscar and one of the Sun Cats, have been in the tank just as long and a bit longer respectively. But they're in great shape. What caused it to die? I want to know what to do make sure the rest of my fish don't die.