Is that a common pleco?
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Would you be able to explain what you did to cycle the tank?
Usually one week isn’t long enough to fully cycle a tank, especially since it seems as if it went from 0 life to basically fully stocked overnight. I’m guessing if the tank was cycled, it was cycled for a much smaller load of fish/life and the larger introduction shocked the system and the ammonia is too high.
Basically just letting the water "air out", letting the water cycle through the filter. No additives / chemicals were added. Thinking about taking a sample of water to the shop and see what they can suggest. I like the notion that you mentioned about it being no life in the tank, to having alot of life all at once. Makes me want to research it more.
I was super bummed when i woke up and saw them dead. I feel bad. I dont want it to happen again
Letting the water "air out" often does not work anymore. Many municipalities have switched to chloramines for water treatment. Unlike chlorine, chloramines are long-term stable in water. This means they need to be removed either by treating the water with chemicals or filtered out with reverse-osmosis or activated carbon.
I think this is the answer. The tank not being cycled is obviously a problem, but not one that will kill off relatively hardy tetra overnight. Chloramines will. You need dechlorinator.