Riven, as I read along the thread, I began to wonder what size tank you have and how much Prime you usually use. You did say you did (or do?) a 50% water change. The amount of Prime you use may make a difference in the smell (that and how you store the bottle, as noted just above). I have a 6G and a 10G tanks. I prepare water head of time, storing it in 3L and one gallon plastic water bottles. I add the Prime as I'm filling up the bottles, cap the bottles and put them aside (like atop the refrigerator to warm up the water a bit). I use 6-9 drop per water bottle. Occasionally after I'm done with the water-bottling process, I can still smell the Prime, though the Prime bottle is capped and put away. At least it's not nearly as bad as cat poo smell
And for me the smell goes away after a short time. I don't think I've ever smelled it coming from either aquarium. I bought a new bottle back in January but have not needed to open it yet. Now I'm wondering if I should check it out.