I'm sure there are many questions regarding fish compatibility and do be honest I've done quite a bit of research, however some advice is always good. I have a 125 gallon completely empty at the moment, planning to upgrade it when the time comes. If you guys could look over my plan for the tank I would greatly appreciate it. 

Planning to dirt the tank: Peat moss, Miracle grow, top layer ???

I want sand for the top layer, and I mean I know it's possible and many people do it, but what type of sand should I use? I'm not really on a budget but I'd rather save some money with play sand than flourite. Does it really make a difference on what time of substrate i use for capping?

Also Fish combinations! I want an Arowana and I definitely want some large, colorful, aggressive fish in there with him. I've been looking for tankmates and so far some good candidates are oscars, red tails, and knifes. What other fish are good with arowanas?

Lastly, what kind of plants should I use? I'm not going to really overplant the tank since i figure they'll need all the space they can get for swimming about but I definitely want some nice aquascaping work to happen. I want to carpet the tank in something grassy (dwarf tears?) and have a few larger plants with some driftwood. 

Any help would be appreciated. Thanks in advance