Hello! My 5-year-old wanted a 'pet' and so we settled on fish. I researched the heck out of it and discovered a new hobby for myself which is very exciting! Everything went so well until our first fish arrived and now they're dying on us

Background:
- Purchased a 25 gallon Aqueon Premium Aquarium Kit (T5 lamp, heater, quiet flow power filter).
- Added aquarium gravel, a few decorations and a bubbler pirate cave
- Planted 2 El Nino Fern, 1 Amazon sword, 1 Umbrella Plant, 1 Narrow leaf java plant
- Filled with tap water
- Added 12.5 ml Aqueon water conditioner
- Added 25ml Aqueon plant food
- Temp is hovering at 78 degrees
We waited one week and then went to petSmart. They tested a water sample and said it was 'great'. We bought 4 long-finned red minor tetras which were supposed to be the 'hardiest'. Float the bag for 20 minutes, added some aquarium water to the bag and waited another 15 minutes, then used a net to move them from the bag to the aquarium. A few hours later fed them a pinch of Aqueon tropical fish flakes which they went crazy over. Also added another dose of plant food (25ml).
24 hours later 2 were behaving strange. We fed another pinch of flakes and none of them went after the food. 2 died within a few more hours. Brought them back with a sample of water which tested 'fine' again although the PH was a little high. Replaced with 2 more. Same story. lasted 48 hours this time but not interested in the flakes. 2 more dead (wasn't prepared for citing life-lessons to my 5-year-old either!).
Perhaps I give up on tetras and move on to cichlids which like a higher ph? was it the plant food? the fish flakes? trauma? locusts? flies? plague.
p.s. i'll be scouring posts next on how to adapt an aqueon water changer to a new pull-down faucet. the adapter it came with doesn't fit. argggh.
signed, what did i get myself into.
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