Hey 6bar - mabuhay pinoy lol
i was also born in the philippines!
sorry to hear you lost two in a short amount of time
honestly though
dont bother getting another until you get a bigger tank
cos they wont last there more than a year even 6 months fi you feed heavily.
the tnak looks small. TOO SMALL
in phil, they put silvers, large silvers in small asssss tanks! cruel, really nasty.
fish looks like it wants to jump out and crawl to the nearest power socket to pop it self into fish heaven.
your gravel looks fine. make sure its not lime based or a calcium based substrate - cos they can leech random things into the water.
but it looks ok as your ph is 6.5.
arowanas can handle ph at 6. maybe even lower for a short time. in nature, during certain seasons, leaves and berries fall into the water.... in asia its ketapang leaves, this makes the water drag, acidic and aromatic, somet hink it triggers breeder.
anyways
you may try to empty the tank, and take out EVERYTHING, wash it and then re set up.
what filter are you running? you said aqua clear 110?
well if i am not mistaken, that is a hanging filter on the side? ..... if i am not mistake and that is your ONLY filter..
then that may be your problem.
You measured nitrite, did you measure amonia?
if you have 0 nitrite its good, but it could mean that the amonia isnt being digested, which means you have too much of that.
Amonia can kill rapidly.
with an aqua clear hang on filter, you will have less than 1 to 2 cups of bio media.
that is no way enough.
For a silver aro you should have a few kilos at least IMHO.
perhaps invest in a external canister filter first and fore most..... that will be more effective....then cycle it - then put the fish in there.
that may be your problem. nitrate and nitrite are the last two components of the bacteria cycle products.
Amonia needs to clear first. so test for that.
some fish are more able to withstand amonia exposure.
hikari sticks or any aro sticks are at least 40% protein which means when your fish digest it, it will quickly pollute the water.
my principle is this
my bio media needs to handle as much load as my fish would eat in a day

LITERALLY, sounds funny
but i have fed my fish untill they actually refuse to eat...and my filter still keeps up.
general rule of thumb IMO...your water in the tank should turn over a minimum 5 times per hour with your filter when it comes to aros.
this is my method

paranoid? well, fish here is expensive in NZ so need to invest heavily on filtration lol cant afford to die. Silver arowanas here cost 70US dollars.
try that and see how you go, good luck.
DO NOT USE UNDERGRAVEL FILTERS
THEY ARE A NIGHTMARE, AND I THINK THEY SUCK