You've got good instincts, its definitely aphids. Here is a previous thread that's got more info on how to deal with it. Generally speaking if you see a bunch of ants its a safe bet that you've got aphids. The plants will recover once you get rid of them.

http://forums.gardenweb.com/forums/load/pests/msg071005279330.html

Thoughts on control? I will hit them with some insectididal soap tomorrow, but will they just come back with if the ants aren't taken care of? I bought some bagged compost from a local home improvement store that starts with an 'L' and I think it was infested with ants.

I have some Spinosad that I bought to hit the ants from the compost; will it kill the ants? It lists fire ants under insects control but doesn't say if it works against the others. I also have some diatomaceous earth that I can try to use, if I only knew how to get it over all the ants.

Thanks,
Adam

Your instincts are also good about the water issue.

If it's continuing to get worse, it's too wet, too dry, or damaged roots.

Ants in general are not a problem unless it is a true infestation.

The only reference to "fat ants" above - was somewhat tongue in cheek, referring to the fact that ants like to farm aphids, so if you are seeing a lot of ants, there is a possibility you have aphids.

Except for extreme infestations, ants have never really bothered my veges.

Those ants will go away if there aren't any more aphids to farm! They steal the honeydew, a sweet sugary substance produced by aphids, and are able to control much of their movements.

In my experience, aphids are among the easiest garden pests to get rid of, and I don't know of any gardener who hasn't had to deal with them at some time or other. You just have to recognize what they are before they chomp up too much of your plants.

Here is a link that might be useful: Aphid Control