while i agree with carlos that your ph is high, it's not unheard of for tetras to thrive in a hard water environment. tetras can be found throughout the amazon river system, including the semi-brackish and brackish delta systems. think about if for a moment, you have had them in your tank, under the same water chemistry for 2 years. i highly doubt that ph is the reason that you are having issues. i would agree that a ph swing and improper acclimation may have caused tetra b's demise, but if your lfs is in the same town as you, they are most likely using the same tap water supply that you are.
as far as softening the water, thise buffers are bad, bad, BAD for the plants. aquatic plants do not handle excessive chemicals very well. if you truly want to soften your water some, i would recommend adding peat to your filter. i know fluval makes a peat bag for filters, and i amsure eheim and aquaclear also make the same. in all my shrimp tanks where i needed low ph levels (5.0-6.4), i always kept a layer of peat moss under my substrate. and my tap is 6.8.





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