but I was thinking, how long would it take for a paima to grow to ten feet? If so arowana cant be miles off, considering they are closely related.
Not as closely related as you might think my young friend.
They're in the same phylogenic order, they're not closely related. The order Osteoglossiformes is roughly as broad as is the order Primates. Comparing a 'paima to an AA is about like saying that a big silverback is closely related to a pygmy marmose. They're vaguely similar at first glance, but at the end of the day, that's all they have in common.
The mistake you've made is understandable, Pole. I don't blame you for not knowing any better; it really was a reasonable assumption. This is why I urge you to exercise caution. If you don't
know that what you're saying is correct, then research it first. That's how you learn. That's how I learned. That's how most of us here learned: research, research, research!
If you know enough to sound smart to people who don't know this hobby and then post around people who do know what they're talking about, you'll say just enough to make a fool of yourself.
After all, the last common ancestor for a 'paima and an AA was alive when the dinosaurs were young and the P-T was still a fresh memory.