Many fish can go with a silver arowana, I have a bunch of cichlids with mine... Some cichlids are gunna be too aggressive, but there are many that will live fine with it, especially if you get ones that stay close to the bottom... or you could get stingray's...
if ur going with an aro and you want other fishes to be in there, get a silver or black aro, because a jardini is very aggressive, unless you've picked out a lucky one, I would say some big cichlids like oscars, GTs, JDs, red devils, those can share a tank with an aro, as long as they are not too small, because you do not want them to become your aros food. Your 125 should be enuff for now, but later on once, your aro grows, u might want to consider a bigger tank.
All this talk about Red Belly Pacu, RTC and now an Aro and O, etc. I see someone setting them selfs up for failure in the long run. You've said you plan to upgrade but you also JUST acquired a 125 gallon tank. An Aro+O+other you'll easily need BARE MINIMUM 220 gallon tank 8ft long (IMO).
Given how fast those fish grow I'd say you'll need to upgrade within 12 month. You really think you'll be ready to break your bank on something over 220 gallons so soon after just establishing this tank? Be practically. There's nothing worse than getting stuck with an overstocked tank and not having the means to rehouse them. Especially after you've grown to enjoy those fish.
aba aba ? not rare but very aggressive ...
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if go with silver 125 g enough for a start, but need to upgrade once it reached ard 16"
If your looking for something small with loads of aggression why not try a large shoal of Buck Toothed Tetra.These things are as hard as nails i've seen a shoal of these totally destroy a texas and an oscar in the same tank its not a pretty sight i can tell you. Good luck with whatever you choose