$150 (mail-order or local pick-up from an online vendor) has been the going price lately, neither cheap, nor expensive (according to Wes Wong anyway and my experience agrees).
What do you mean when you say the first tig died because of "good ph"?
Is your acrylic tank on foam? One picture indicates a floating corner on what appears to be foam. Not msging you to be a prick or anything because your tigrinus is absolutely beautiful, but I would suggest getting that foam out of there and make sure the entirety of your bottom is supported by plywood. I am currently dealing with the exact same mistake.
Hope this guy grows well for you, enjoy!
Tigs and Brachyplatystoma in general are not as hardy as other other large Pims but overall, they are pretty resilient.
Outside of common stress factors (ammonia, nitrite, inadequate DO, too hard water, too high pH, unstable temp / pH, bullying, overfeeding, etc.), IIRC, I've only encountered a hypothesis by T1KARMANN that seafoods kill tigs, or perhaps more specifically marine crustaceans.
If you'd like, I can dig out a link.
I see ppl use foam on top of the plywood and also does Joey DiY