You might want to consider pool filter sand as a substrate because: It's relative cheap compared with gravel, it's prewashed so you can dump it right in, and best of all, it does not hold dirt like gravel. I have it as substrate in a 650 gallon fiberglass vat and it has been set up for about a year. The sand has never been cleaned even once and still does not need cleaning. Cleaning the filter and regular water changes are all that is necessary. In a tank your size you could spend hours trying to keep the gravel clean and it won't be easy in a deep tank.
Plants may be a problem since you tank is deep and they need strong light. If you go with floating plants you could use water lettuce, water hyacinth, riccia or salvinia depending on what's available and legal in India.
I would not leave the light on 24/7. It's not natural for the fish and can actually cause problems for plants as well. You could always have a dim light come on from dusk to midnight or whenever you go to sleep if you want to see the fish as night and leave the bright growing light for the day.
Highly reccommend a lexan or polycarbonate cover for the tank so the arowanas don't beat their heads against glass or jump out.