I took my weekly journey to the "better" fish store in town (the "best" that also secializes in plants is a lot farther) just to get familiar with the fish there, now that I'm looking at freshwater fish and not goldfish.
Ended up taking home 3 Serpae tetras and a skunk cory. The guy there really thought that my water hardness and pH wasn't a problem for these fish and thought the fizz tablets (and even adding CO2) were not necesary for plant growth. This is not what I read, but I'm gonna give it a try.
So, before adding the fish I decided to do some water testing as they floated awhile. My pH is back to 7.8, same as the bucket I have aging in the bathroom, and the KH has skyrocketed to 13 degrees, from 7 or 8 of aged water. So, the tablets may have added CO2 the first time, but with the KH now so high, I don't think I dare to add a second tablet.
The seltzer smell is all gone. The plants are very, very green. I added some regular gravel freshly rinsed, to cover the sharp flourite. Followed precedure for acclamating the fish to temp and water, released them, and two were dead wthin the hour.
The fish store thinks it was the fact that I disinfected the filter box with bleach after transferring it from the goldfish tank. I rinsed it well, let it dry for a couple of hours, added Dechlor straight into it when I added some extra to the tank before addiing the fish, used a fresh filter bag with a chunk of old filter inside.
I think it was the KH being so high. So, I got almost all of the water out of the tank, filled with aged dechlorinated water, and added the cory and the single tetra back to the tank on the advice of the store. Actually, he wanted me to not dump the tank, but I couldn't bear to kill the other fish on purpose.
This is getting discouraging. The goldfish was so unhappy I put him back in the pond. A bunch of empty tanks, how depressing.