When they are dying they do not smell at all. When they are dead they decomp rather quickly depending on their size and the size of your tank, yes they can create major issues with ammonia levels. They have the same effect on your tank as a fish would if it were to die. Their downside is they decompose faster and not many things eat them. So... as mentioned keep a watchful eye, if it appears to be melting away things stringing off of it, take it out ASAP. And yes once they start to decomp... the smell worse than any dead fish I have ever encountered. Good Luck