Put a block of fd blood work in a net and run it under water to wash it out, this will stop it polluting the water. Put a pinch of it on top of the water. He should start eating straight away. Leave the guppy in there. He will be more scared of the Arowana and stay out of the way.
I can see some plants, thats good. I would get more though. If he does get scared he will want to hide in something rather then be out in the open.
Light is good... it promotes growth. Try not to keep him in the dark all the tme.