pleco died

So I just did a 50% water change in my tank and put in a new plant, rinsed the rocks and put in fresh water. I have quite a good filter and the silt from stirring up the gravel was gone within 30 minutes. I have 3 fish in the tank for the time being: a 4-inch goldfish, a 4-inch albino...

I am new to the hobby and I have done a lot of research on keeping fish but I just can't figure out why my little pleco died(he was on the top of the tank when I woke up this morning). His belly wasn't sunken in and I saw him eating last night after I turned the tank light off, he was quite...