Red gunk inside my filter, what is it? 10 points to best answer?

The fibers that compose the filter pads are quite susceptible to absorbing any strongly colored particles. Parasite Guard contains acriflavine, which will give the tank water a slight tint, but will concentrate in the filter media, producing the charming mess you've found. The smell is likely a result of several factors. First, filter media will not smell pleasant, simply because it is the main location in which decomposition occurs--as solid wastes get trapped in the filter, they decompose. Second, fish tend to produce more waste when stressed, and the stress of infection and medication can exacerbate the stench. Finally, most medications aren't all that olifictorily enticing, to begin with.