yeah you lost your cycle when you swapped filters. If you are unfamiliar with cycling an aquarium I highly suggest you research the topic. Bacteria is in the filter media that breaks down toxins produced by the fish. This bacteria take time to establish, 4-6+ weeks often. Without it the toxins build up in the water and kill the fish. Not saving the media from the tetra filter caused a large loss of bacteria resulting in fish loss. Your nitrite at 1ppm is high, easily high enough to kill gouramis. It should always be zero, since it is toxic to fish. The presence of nitrite almost always happens alongside an ammonia spike, which again is toxic and can easily kill your fish. The ammonia spike comes first since it is produced by the fish with the nitrite following. Your test strips do not test for ammonia. I would recommend getting an API liquid test kit its what many people use.