Help, piranhas not eating

[Jaws]

My piranhas haven't been eating on a consistant basis like they used to in the past and don't eat as much when they do eat now. I thought that it might be the frozen bulk shrimp that I was feeding them because it had really faint brown patches on portions of it, but then I looked around and everywhere that was selling this shrimp had the same look to it. I just didn't notice it before I guess. I know that piranhas will sometimes go for a period of time without eating, but I can usually tell when mine are hungry because they start fighting with eachother a little more. They don't appear to have any illnesses but I wouldn't know what to look for if they did. They look as healthy as they always did.