Erised
Cheer up, the worst is yet to come. ~ P. Johnson
So it could be the cory's? But they seem so peaceful! My yellow guppy used to be resting alot but the blue one used to swim around alot.
I was thinking about getting rid of my Zebra's anyway because they always get the food and my gourami and cory's haven't had any since I got them. For the cory's do I need to get the special bottom feeder food?
It won't have been the corydoras, they are very peaceful bottom feeders, honestly don't think of them as a suspect at all. It was simply wrong information given out by a lfs. Getting rid of your zebra danios is ofcourse up to you, though I love them to bits. You do really need a shoal of at least 6 like The-Wolf said, because else they can get nippy towards other fish, where as in a larger group they tend to hang around and chase eachother rather than bug other fish in the tank.
Corydoras, like all bottom feeders need special food that sinks. You can get catfish pellets/tables in nearly all fish shops, so have a look for them. As to getting around the danios eating all the food, that can be a bit tricky I got around it by pinching the flake food inbetween 2 fingers and bringing it down to about halfway the tank before letting go of it. This way it's pre-soaked slightly and won't float up to the top. The current of the filters in my tank usually catches the food and blows it around the tank, causing all of my fish (apart from the bottom feeders usually, which is why they need sinking tablets/pellets) to be able to get some.
--Edit--
It could very well be the gourami as well, they too aren't known for their loving temperament. Either way, be it danios or gourami, I find it strange the guppies died from it. Nipped tails do not kill. It could have been the stress from being chased and nipped that killed them, but usually they'd get ich or something first, which makes me think the guppies probably weren't all that healthy to start with.