I would worry more about why they are not eating. Its a miracle if the food in my tank lasts 4-5 minutes. Something isn't right here. Beef heart will foul the water quickly and escalate whatever your underlying problem is, I wouldn't suggest leaving it in more than 10 or so minutes if they do not eat it.
Are the fish new to you? Have they always been slow eaters for you? How big are the fish?