Welcome fellow ]|[onsterfishkeeper! this site will give you all the answers you need...
aside from what has already been mentioned, 7.0 is the ideal PH for an aro, 30-40% WC(water change) once a week,nitrates below 10ppm, aros love slow current so addind a powerhead would help not only for exercise but would also prevent potential gillcurl in the future (make sure the current is not too strong though it will just give him stress)
if your new aro is between 4"-10" now would be agood time to train him to eat quality aro pellets and MP(Market Prawns) to ensure a well balanced diet and that he gets the proper nutrients needed for his quick growth (like protein & B-carotene abundant in the exoskeletons of shrimps - just make sure you remove the head and tail but if your aro is way too small for whole shrimps then peel & chop them up)
just make sure the waters clean and that no other tankmates are currently nipping on his fins and it will heal in no time... you may want to add a lil rock salt to prevent bacterial infection and speed up the healing proccess
start saving up for a bigger tank, everyone's right...you'll need that in the future
Superworms for snack in between meals for added protien
PM hao if you're not sure about something

(he may be 12 but don't let that fool you, this guy has arowana blood coursing through his veins so he knows alot...lol)
ALWAYS make sure there's a lid on with glass cover atleast 3/4" thick(chicken wire will work too) # 1 cause of death for an aro is jumping out and drying on the floor or wherever it is they land on out of water
HAPPY ]|[ o n s t e r f i s h k e e p i n g ! ! !