Hi there
everyone has to learn, unfortunately, sometimes learning means dead fish or sick fish.
keep the tank at 28 minimum celcius, water change 30% should bring that down to 25 and go back up quickly. depends on where you are in the world, if you are in asia, water should be warm ish coming out of the taps anyways, here in new zealand, tap water is da,mn cold!
your tank is small and will be out grown by a silver easily. asian, maybe it will take some time but the filtration is seeming small! so change that
also, 2 weeks cycling isnt enough
what i did before hand, i put the filter with all the media, i hope you have biomedia, into a established tank. then i over fed those fish abit untill the filoter is cycled for one month minimum, then put it to the new tank and introduce fish. OR i swap media from different filters into the new ones. Lucky i have 8 canister filters to work with.
in your case, you need to cycle the tank, i this fish dies, then before you get another, first get a bigger tank, then put some cheap goldfish or cichlids in there, maybe a catfish that is hardy and eats alot.
get a bigger filter, and feed the crap out of hte catfish and change water every 3 to 5 days.
then after 1 month, your filter should be cycled.
get a test kit also
as a note: Sometimes when you feed an arowana and its full - it seems very sluggish and slow, like a human that consumed great amoutns of food. At times they may seem like they are sitting at the bottom or just relaxed and kinda not even swimming and just cruising slowly.
but thats providing they dont vommit food and have good filtration.
both my aros, after eating massive amoutns of food, sometimes just float at the top or drift along the bottom of the tank untill the belly digests some of the food, then it swims around lookin for more food
