Jaden wrote in :

As as matter of fact, I live in a very hard water area. I think I've
found my problem (such as it is). I'll buy an acid buffer this
evening.

Just be careful using it. Do not over dose. Try to drop your KH gradually
over the course of a week or so. Dose with Acid Buffer maybe three times a
day many hours apart. Too much carbon dioxide in the water will reverse the
oxygen affinity of hemoglobin and basically asphyxiate fish.