Since you have a cement floor you could do a concrete style tank, there’s a thread regarding a 15,000 gallon concrete tank floating around on here.
Turns out it’s 5k
https://www.monsterfishkeepers.com/forums/threads/5-000-gallon-arapaima-gigas-tank-by-johnptc.48640/
That's a beast. I would totally do that if I could convince my wife. Lol