3 channa pleuro, 2 bichirs (del and endli), 3 indo datnoid, 1 spotted gar. I've just introduced my new leichardti into my tank yesterday, and the leichardti started fighting with my group of 3 channa pleuro.
100 gallon tank, may be it's really too small
I think you answered your own question. I understand you live in Vietnam, so they may have different ideas on stocking there, but you can't overcome the fact that even with incredible filtration and water change schedule, you are completely and utterly overstocked even with your current setup. There is no way to get around the aggression/territory issues in such a small tank. I assume for 100G you have a standard 6'x18"x18" (183 x 46 x 46 cm) tank. I am assuming this is your biggest tank and you want to keep these fish long term in this tank.
It's like cooping up 9 large murdering psychopaths in a small public bathroom, introducing a 10th, then wondering why they are fighting amongst themselves. Even peaceful fish will get frustrated in cramped, overstocked quarters, let alone aggressive ones.
Also, a silver arowana when grown has even less chance of a leichardti of fitting into a 100G. It literally will not be able to turn around as the depth of the tank is much shorter than it is long, and will get gill curl and die a horrible cramped death. A silver arowana deserves something like a 8'x3'x3' AT LEAST (preferably a pond). Please DO NOT put the silver aro in your smaller tank as you are suggesting.
My recommendation is to either get a much, much larger tank if you want to keep these fish together (we're talking 300G, and even then, you may still have aggression issues but less likely), or return/rehome the arowana. You will need a superhuman effort to keep your current fish alive, healthy and to their full potential/age in your tank as it is without making it impossible.
I'm sorry if I come off harsh, but the reality is that no matter which way you try to look at it, what you want to do with your current 100 G tank is impossible without constant aggression and poor fish welfare.