Hyla Cinerea died, I need to know from what

[Froggo1]

Im going to go into detail, so get ready to read a bit

A few months ago, i purchased my first frog, an American Green Tree Frog, from my local Petsmart. Usually i wouldnt do this, but this Petsmart keeps their animals in very good conditions.
So of course before any of this i did copious amounts of research on my soon to be friend. I saw diet, enclosure, and everything else. I eventually got what i believe was a male frog, due to his small size.
Later at home i saw he was pretty small, and thought he must have not been that old, or he may be mal-formed. But i went along with it. I enjoyed feeding him, and watching him explore his large 10 gal tank. I would keep it at around 80ish degrees F, 60% humidity, and near my window. The only thing i didnt do, was get a UV light, as i had read that they werent necesary from many sources. So time went on.
About a month later, he decided to stop eating. This worried me, but i remembered that when they moult, they dont feed, but this went on for an entire week.
Again, i didnt think much. About another month later, i got my second frog, the one i currently own, who is much, much bigger, about 3 inches. I put it in quarintine for 1 and a half weeks before uniting the two.
Now we get to this last week. I had noticed that the first frog, who i named Pringles, started getting discolored. Now i was worried, and, he wasnt eating as well. Then 3 days ago, he completley stopped, and was very gray, like ghost gray. Yesterday morning, he was on the floor of the terrarium, belly up, and i was terrified. He wasnt really moving, and his breathing was off. I separated him from Soloman (the second frog) and watched him, as i desperately tried to find answers. Sadly, i found nothing, and i had to put him down with Orajel, but before i grabbed him, he rapidly jumped in the holding cell, and immidiately after fell to the bottom squeaking, with its legs streched out and his arms on his chest, belly up. Soon after i put him down and buried his body.

I need to know if this has happened to anyone, or anything similar at all, its the least i can do for my amphibious comrade now...
Please help me find out what happened. Did i do something wrong? Was he already doomed from the start? Anything helps..