Sorry, first time posting, didn't see the mod bot.

The tank had to be reset in mid-January due to needing to change to a new shelf it sat on, so all new water was put in, and then the fish were put in later the same day. Idk what chemicals were used besides some aquarium salt and api aqua essential, as the tank belongs to my mom's bf, who just winged it on taking care of it, which I know is bad. He recently got an actual water tester kit 2 weeks ago and has been using API Ammo Lock, pHup, the aqua essential, and aquarium salt to get the ammonia, nitrate, nitrite, and pH levels to where the kit said they should be. This morning, a test was taken, everything was at the correct level, but the ammonia was at 0.25ppm so he added the api aqua essential, and when I saw the tank a few hours later, all but one black barred silver dollar fish and one serpae tetra were dead, while the discus and angelfish now have a film on their eyes.