Know this is old but I just had the same issue so I started searching tank odor. Set up a 125 very recently and it smelt fine this morning, a little earthy. Fed the new fish some massivores and came back 3-4 hours later to a brutal smell to where i had to do a 15% water change and removed the untouched pellets. I was thinking something was really wrong and all the tests came back fine, good thing I just bought a jumbo bag of pellets ?