The pacu will need at least 300-350 gallons of water and 3/4" glass.
No a 125 does not have glass thick enough. Neither does a 210. Mine broke the lid on their 210 in a jump. The tank was made with 1/2" glass, the lid was 3/8" . The sides of the tank were only 1/8 of an inch thicker than the glass that was broken.
I just measured the glass on my 125, its 3/8".
Its as simple as this.... your pacu WILL break that glass. When it does, you will have 100 gallons of water on the floor and however many fish that are in that tank dead.
Do you really want to add more fish to the death toll?
Either find a better home for the pacu, or save your dollars and get ready to buy a bigger tank, or a pond. FYI they do a lot better in the company of their own kind, in pairs or trios is about all anyone can realistically house, but a pair of them is a lot better than a single.