The weiners definitely are bad for them, no question about it.
It might seem harmless because they enjoy them, and in fact there are some dietary benefits---but these are outweighed by the large amounts of mammal fats which they contain. Fish aren't really designed to digest and break down these fats as we humans are, and ingesting this type of food regularly can lead to strain on the liver and all sorts of related health problems.
For this reason, it is recommended that you stick with foods that are designed specifically for your fish (e.g. commercial pellets/flakes). Treats should consist of fish-based offerings (such as frozen fish, shrimp/krill, etc.) whose fat content is much easier for your fish to digest than the type of fats contained in hot dogs.
BV