If you can, you should pick up a freshwater master test kit from API. Not sure if you can get one where you live but it will help you keep your parameters in check and it is much more accurate than strips. It costs around 20-30 bucks here but is well worth the money. You should aim to keep your ammonia and nitrite at 0 and your nitrates under 40ppm. With both fish that may be a problem.
It's good to do a 50-60% change once a week like hendre mentioned until you can get a bigger tank. I would consider possibly selling your arowana. They really need something like a 300+ gallon tank or a pond to be happy. Maybe there is a farm or garden store nearby, they have large plastic or metal stock tanks that would work as a pond if you want to keep the arowana. I think a 2.5x2x2 is around 60 or 70 gallons?
I think the food you are feeding him should be fine. Hikari and Omega One make really good cichlid food too if you can find it. You can also use vacuum sealed frozen bloodworms as a treat, he will go crazy over them but feed those sparingly because they are fattening. You should also try feeding him a few plain frozen peas every week. Like others mentioned they get prone to bloating. Peas will help clean him out. If he does seem to have a parasite there is a sticky on how to treat it in the health issues section of the forum.
Also, most cichlids like flowerhorns can easily live to be over 10 years old! You will have him for a long time.
Welcome to the forum.