I just can't figure it out. At first it was because I had too many fish in my tank but now I went with smaller hardier fish, and I just bought them on Thursday and one(the most beautiful I have ever seen or had) has died!!! I found him stuck to the intake of my filter but he was still breathing. I nudged him off to see if he would swim and he tried but was swimming upside down and kept trying to right himself but gave up and sank to the floor!!

He is gone now. He was one of those brillant tiger guppies that have the blueish/grey/green spots and stripes on his body and a huge tiger striped yellow and black tail.
Before these set of fish I was told that I needed an air pump to put oxygen in the water because my filter discharge was not breaking the water's surface.
Then the cold set in and I bought an automatic heater to keep the temps around 75-76 degrees.
I took a sample of water to petco and they tested it and it was fine. I bought 2 guppies.
Those 2 did great for a week and I was told to come back with a sample of water to see if it doing ok. This time the nitrates were just a tad above normal and was told to only feed every other day to bring them down. So I did. He told me it was ok to get 3 more fish. So I got 2 more guppies and a tiny Pleco.
I waited 2 days to feed them because I fed my other 2 the night before and these 3 just got fed that morning. After the second day I decided to put an algae wafer in for the Pleco and I knew the guppies will eat it too so that was all I put in. About 4 hours later all the guppies were fat so I figures they helped eat the wafer. The water was great with a tiny bit of cloudiness due to the wafer I imagine.
Now this happens and every other fish is doing great as usual but this fish dies?? What gives??
I have noticed that he separated himself from the other 3 and just hung out at the top of the tank looking depressed(dorsal fin flattened and tail not fully fanned out but it wasn't all the way collapsed like fish do when they are having a bad day.
Now there were 2 of each, 2 males, 2 females and the Pleco.
I was told by several people that having that many fish was fine because of the small size of the guppies and the Pleco just sits around out of the way just stuck to a wall or what ever he feels like sucking on!
Water temp was 75 when I found him and the light was off for most of the day.
I only have a 5 gallon tank and thats big enough for me and my wife. We can't go any bigger.
Please help!!!