Chiq,

There are several things that can go wrong with seedlings. If they are getting enough light and adequate moisture, without seeing a picture, I would guess that they either are suffering from damping off or root rot. These two things can both be caused by the same pathogens and can occur as a result of overwatering and/or not using a sterile soil mix. Damping off attacks the seedling at the soil line and root rot attacks the roots. In either case, the plants shrivel and yellow because they are no longer being fed from the roots. Dig up a couple of the seedlings that are failing and inspect the roots. Healthy roots are white and firm in most plants. Rotting roots will be dark and mushy. If this is the cause, it is difficult to save the yellowed seedlings, but a fungicide treatment may be helpful in protecting the healthy ones. The ants may be a problem if they are feeding on the roots, but sometimes plants can coexist with ants. Take a look at the roots and report back here. Also, tell us about the soil you are using. Is it a commercial mix, something you made up, or just dirt out of the ground?

--Bob