I don't remember the exact setups at this point, but from what I recall, for the Tarpon, I was running a 50lb match rod setup and for the Arapaima, I was running 68lb or so bottom rod. I'm still working on leveling up and the bit rate for young/common to lower size trophy is higher, so I set my tackle up to get the most xp from those size fish and most often fish from shore with multiple rods.
What happens, though, is that a unique does bite occasionally. For the Arapaima, using the 68lb bottom setup, I fought for 2 IRL hours and the fish never got tired so I could never reel it in. My leader finally wore out and broke or I would have continued to fight it. This actually happened a second time with the same setup several days later, and at the 2 hour mark, I just broke the line and walked away. The only reason I didn't fight the Tarpon mentioned above for longer than 30 mins is that I recognized what was happening and just broke the line.
Honestly, I have no problem fighting such large fish for long periods of time and that really isn't my complaint. That is what would happen in real life and should be expected. However, what seems to be happening in game with the Tarpon and Arapaima, specifically, is that they never tire out. At some point, even on under-powered gear, the fish should get tired and I be able to real it in. I know they are different types of fish, but I have I've pulled in unique Mirror Carp and unique Flathead catfish on under-powered gear in under 30 IRL mins. My typical Flathead setup is a 30lb bottom rod and my typical carp setup is 54lb carp rod.