Hi everyone, this may be a dumb question but here goes. I have a 55 gallon tank with 4 2 and half inch africans 2 firemouths and 2 green severums. my questio is this, my fish swim around nicely and seem very happy but from time to time they have strings hanging from there anuses. I know it could just be dung but it doesnt seem to fall off very easily. As in hanging for 2 3 hours sometimes. IS this a problem or is it natural?