Fluttermoth: I'm using the API testing kit. And no, I always use sprays outside my room as I have another tank in it. Thanks for your response!
Star4: Yep, did so slowly and gradually with the water. Maybe it is pH shock, but I did everything you said to avoid that.
I purchased a 40l Panorama tank and set it up roughly 4 weeks ago. I added the usual: tap water conditioner and bacterlife and let it sit for a week. I tested the water and everything was perfect so I then bought 2 penguin tetras (now aware that was a silly choice) and they died the next...