Why has my chicken got diarrhoea?

If your chicken has diarrhoea it could be down to something they ate, or possibly a heavy worm burden in their gut.  Initial advice would be to remove treats from their diet and feed only layers pellets and add Nettex Herbal Gut Conditioner to their drinking water to help maintain a healthy digestive system.

You should also take a faecal worm egg count to see if your chicken needs worming, and treat appropriately with a licensed wormer if so as a heavy worm burden can cause rapid weight loss.