Thanks, all! Appreciate your suggestions..
Kethlas -- yes, my number 1 suspect is the Flourish & Excel. I've rinsed out my tank and won't be using those again....
Creat -- yes, I think I had a mini-cycle after adding fish & 2 RCS Shrimp. But the ammonia never even reached 0.25PPM; surely not poisonous so soon...? I added my second Betta after a 50% water change -- surely should have kept it OK for a few days...?
OFL -- yes, brand new tank, rinsed under tap water. Gravel, also under tap water. Cleaned only with water rinse and clean aquarium scrubber -- no soap or other cleansers... Water/cycle tests using liquid test kits from API and Tetra. De-chlorinator is Tetra Aquasafe, exp date 07/13. I tested my tank after water change but before adding second Betta. Ammonia 0, Nitrite 0, Nitrate < 12.5PPM. I understand the numbers could have crept up, but the fish died within hours
. I suspect you may be right re: PH, etc. being different from shop source. But I've bought all my fish from the same shop (Fintastic) and had only 2 casualties before -- both bumblebee platies.
I use the same tap water and de-chlorinator for my main tank (neons, platies, 1 guppy, 1 pleco) and my QT (now full of baby guppies, long story) and I haven't seen any problems. I thought Bettas were said to be hardy fish...?
OFL, I'll check my water, as you suggest and post results. BUt I keep suspecting something contaminated my water.....
Hoping my most recent rinse-out will solve the problem...
Is there any way the Mopani wood could have caused the issue? I got it from LFS, so believe it should be OK... or the gravel...?