Hi. I am not a fan of caging any living thing so I have never had an aquarium, but a neighbor moved and left her one fish so of course I brought it home. The aquarium is a 5 gallon hexagon shape and contained one fish that I researched and found to be some sort of tetra. Thought he was probably lonely since they are schooling fish so went to petsmart and looked for fish that looked like him but found none so I bought 2 neon tetra's and a guppy. Everything going fine, cleaned the tank according to petsmart advice. The last time I went to clean it I unplugged the filter as always and when I went to plug it back in it wouldn't work. Hubby said it wouldn't hurt the fish, just make the tank dirtier faster. Boy it got dirty fast. 3 days later I wake up and one neon is dead. Panicked, went to petsmart to buy a new filter. Didn't have the one that goes w/ my aquarium but was sold another one to work on aquariums up to 10 gallons. Brought that home, cleaned the tank again (I only remove half the water every time) but still murky. 2 days after new filter put in, guppy is dead and other 2 fish don't look too good, colors fading, not hungry, etc. That was 2 days ago, tank has cleared up, fish look better, but I'm wondering what is happening to my fishies?? I don't want another funeral! What else can I do to make sure they are healthy and happy? Also, aside from all this, I soon realized that the neons were not going to hang out with the lone bigger tetra. Will schooling fish only hang out w/ others that are just like them? Thank you so much!