Most arowana grow up to 2 inches per month for a long time. Yours could be slower, but he's not going to slow down unless tank conditions become poor.
Right now, I'm guessing that you are changing about 75% of the tank water per week. Otherwise, you may have higher nitrates than you probably want. In two months, at your aro's current growth rate (which is about +1.3% per day), it will be 12 inches and the amount of nitrates added to the tank each day will have doubled.
In 2 more months, the aro will be closer to 15.5 inches at his current growth rate, and the nitrate levels will have doubled again. Ideally, you'll be doing 75% water changes every other day. That's still using a slower rate of growth than many people report an aro might reach.
A 15 inch fish in a 36x18 inch tank is not going to be a good choice, imo, so you have only a small amount of time to solve this.
Removing the other fish doesn't really fix much (in my calculation) since I assumed they would not get much bigger.
