can adding regular aquarium gravel kill fish?
My wife has a tank that is just hers. She only really likes neon tetras, so it has a five tetras, a cory cat, and a kuali loach. It is 20 or 29 gallons, not sure as it was freebee. Anyway, the tank was up and cycled. It had an undergravel filter when my wife decided she didn't like the color of the gravel and had me take some out and add in a bag of black. I also did a 30% water change. Within a few hours most of the tetras died. I couldn't figure out why at first because it was pretty instant, but then I checked the levels and the nitrites and nitrates were high (I still find it hard to believe it happend that quickly) so I think it did a minicycle? I also have a piece of driftwood in there so I thought that maybe the pH had lowered and I changed too much water (we have hard water) and it shocked the fish??? Still don't know.
Anyway, upon recommendation I added a hang on the back filter. The nitrates and nitrites leveled out. We added back new tetras and they were doing fine. Had even done a few gravel vacuum cleanings and WC of 10%. My wife was complaining that I didn't add enough black gravel, and it was thin in a few places. So today, she added another four pound bag of black gravel. I washed it this time to be sure it was cleaned properly. Same thing, we almost instantly lost 1 tetra and the others seem stunned, but haven't died yet. Didn't change water this time. Checked it and the nitrates are a tiny bit high, but nothing out of the ordinary from what they have been at the last water change. Everything else is the same.
Can adding regular aquarium gravel kill fish? What am I missing?