My dog ate a bit much, had boneless fish yesterday, now has diarrhea, and its gut is usually weak

Dog diarrhea has many causes: overeating or wrong food causing indigestion or gut flora upset, chilling, gastroenteritis, or parasite or infectious infection. First fast from food and water half a day to a day, give probiotics and antidiarrheal, then small easy meals, and deworm regularly to prevent infection.

If diarrhea continues with low energy, poor appetite, fever, cough, or vomiting, take it to a vet for diagnosis and avoid delay.