Iradescent Shark

Question
My Iradescent shark is very solitary and anti social.... I also had a bala shark in this 2.5 gallon tank. they are relatively small but the iradescent shark was a bit bigger than the bala. I came home today and the bala was dead. When i fed the fish this morninng it seemed the iradescent shark was eating all the food so i kept putting in food (tropical flakes) so that the bala coud eat. and when i was confortable with the balas food intake i stopped.. I didnt put too much in thogh (didnt wan to cloud the water). So i took it back to walmat and got another iradescent shark. This one is lively while the older one is not. CAn you tell me why this is?  i also have a "Smallworld Pump & filter kit" which only filters harmful gases odors and discolorations. shoud i have gotten a diffrent filter that Suctions the water rather than this submersible version? Did i do any thing else wrong with my fish care?

Answer
Your first mistake is that you bought Wal-Mart fish.  
 I have have several fish, have for sometime, with good luck, but have not been able to keep either one of those two fish alive myself!  You picked hard fish to deal with.
If I was to say what happened, I think you have to small of a tank, those fish are very active, they need more room. They also get big.
You need a different kind of fish for that small of tank. Good luck!