Are my plants dying because the water is too warm?

I planted a handful of vegetables in a pot on my patio. After a week, they were sprouting and all was fine. Now several weeks later, they're wilting and I'm wondering if it's because the fertilized water I use to water them was outside, in full sunlight at times? That's really the only variable that has changed I still water them every day I just used to keep the watering can w/ dissolved tomato food out of the sun.....