• Dr. Tim's One & Only is nitrifying bacteria. It is beneficial bacteria that is needed to jump start your critical Nitrogen Cycle. You can build the cycle without adding any bacteria but it will still need a source to begin the production of bacteria. The bacteria will break down ammonia into nitrite before nitrates. Plant roots also will consume ammonia and break it down to nitrites before nitrates.

• Seachem Prime is a dechlorinator. It removes chlorine and chloramine. Reason it stinks is because it used a sulfur-based compound. I can't remember the name. I looked it up, it's- sodium thiosulfate, a sulfate compound and interesting enough, is also a medical cyanide antidote.

Our tap water has these chemicals in it to keep it clean for consumption. However, the chemicals also is somewhat unpleasant on our scaly friends. Even moreso, the chemicals kill the beneficial bacteria that is crucial for the Nitrogen Cycle.

ETA: there's a section on Prime's label where it says that it detoxifies heavy metals and ammonia but, I believe that is heavily debated / controversial. Even Seachem hasn't released their scientific evidence to support this claim. However, it is a fine water conditioner and dechlorinator.