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Early on this week I bought 3 1" long platies from a local chain store for my 20 gallon long. This tank has never been used before. However, it was cycled, and stats were perfect (NO3=20ppm, NO2=0ppm, DH=50ppm, KH=120ppm, pH=7.0). The temp is 76 degrees F. They were kinda test fish, I was planning on keeping them on the long run, but I wanted cheap fish that, if they died, wouldn't cost too much. Well, I've been away all weekend, and when I came back, I found 2 dead. My mom also told me that she never turned the light off. So I'm wondering if it is possible for fish to be so stressed out by the 24/7 light that they would die. The parameters are still the same (3 tiny fish weren't enough to change it), so i severly doubt my tank had anything to do with it. There's also a possibility they were sick origionally, but I made an effort to pick the fish from the only tank that didn't have dead fish in it. However, I have had horrible luck before, including mysterious heater problems, some strange disease that litterally disentegrated my fish after feeding a cube of frozen bloodworms, and serious pH problems (which I overcame by increasing buffer). Is it possible that the light killed the fish, or is it something else i should treat for?

I have left a light on for days at a time without any deaths. I wouldn't recommend it often, but I doubt that it caused the deaths.

Fish included in this would be dwarf puffers, dwarf gouramis, neons, von rio tetras, asellus puffer, glass catfish, upside down catfish, hatchets, cherry barb, brown knife, clown knife, and many other fish I can not remember over the years(+many African cichlids).

Something else was going on either with the fish or tank. "Substitute" caretakers often overfeed, be sure you leave them with clear instructions on the care of the fish.

Could be anything but it is definitely possible for fish to die from leaving the light on. It will at least increase stress so something else kills them easier. You should always provide several hours of complete darkness in a 24hour period. Simplest solution would be to get a light timer for when you can't be around. It only costs a few dollars and a few seconds to set and plug in.