Monitor the fish for the next 72hours and let us know how they do. My concern here is that your aquarium could have suffered a sudden spike in ammonia or nitrites due to the LARGE fish load that you added at once. It would have been going along fine and reached a "balance" in the nitrogen cycle but once you added the new fishies the bio load caused havoc and your filters bacteria could not cope and handle the increased bio load. This is likely what happened as you are saying there's no signs of disease. Its really important to ID the cause of fish losses before blaming the LFS and even my first reaction would be to name and shame but first make sure your aquarium setup is not at fault. Filter media appears correct. If you're not going to be adding plants, then you dont need to worry much about the lighting - it should be enough to view the fish and a standard plastic 4ft light fitting and T8 tube should be fine. As the others have said, Algae will continue to be a problem for as long as there's no live plants in your tank to use up all the nutrients thats feeding the algae. Dont let the tank get direct sunlight and keep the lights on for less time to slow the rate of algae growth. Perhaps consider some live plants, once they're in the water you shouldnt have any allergy issues - just be sure not to get any plant that might flower once it gets to the water surface.
Welcome Iqbal - I am afraid I must join in the chorus - Add live plants and lots of them ASAP - BTW The Arowana will eventually grow to eat everything in the tank, and if the next door neighbours kids walk past, it will eat them too !!!!
Hi All, Sorry for not updating sooner. Everything seems lekker in the tank. Will put new carbon into the canister this evening. Having spoken to one of my buddies (certifiable genius), and reading one of the posts (can't remember which one), I did not feed them yesterday. I'm hoping to see a lot of activity this evening. There is the possibilty, that one or two of the smaller fish may have "disappeared". I'm obviously gonna have to step up my water cycling routine. I still can't believe I've finally gotten an Arowana.