Is that a normal practice where you live?
Anyway if you are only keeping them for 1-2 months each, you will not have time to "understand how to care for fish".
You should read first, in order to understand the basics (like temperature). Then, gain some practice having appropriated fish for your 60lts (16g) tank. Small sized species of fish (meaning fish that stay small once they are adult and not fish that are small because they are babies but will grow....)
Then, in a year or so, when and if you upgrate to the 250 lts (69g) you mentioned, you will be able to have medium sized species of fish (perhaps the parrot, I don´t know much about them).
In some years to come IF you upgrade to a big aquarium, you may think about the other big fish you mentioned (specially the arowana).
Take your time and buy fish appropriated to the aquarium you have at the moment. And keep them for as long as you can. Ideally until they die of age; that´s the only way to really learn about keeping fish.
Holding them for 1-2 months and them trading them for others is not "fishkeeping"...
