everyone can always tell u bigger, but buy what you want. a 150gal tank is plenty. especially one that is 20" wide. or 19.68 inches to be exact. buy yourself a baby arowana, 2-5 inches and enjoy it. try to upgrade later if you can, or plan ahead n maybe try to find a keeper in ur area who may adopt it a year or 2 down the road. my 20" wide 150gal tank currently houses my 22 inch leichardti. before i had that, and before i bought my 400gal tank, i had 2 silver arowanas each over 20 inches, and a jardini about 22 inches.
of course people say put yourself in his shoes, but he'll be fine for at minimum 1.5 years. just feed him every other day as a youngin, then as he passes 12 inches feed him once every 3-4 days. it'll reduce the supposed 1-3 inches a month rule cuz people always stuff their aro's daily and it speeds up the growth dramatically.
everyone has an opinion, you asked about aros and you were looking for answers. instead you got people telling you to buy some other weird fish, and avoided your question entirely. the choice is yours, the answers are on the table. n then there are the opinions