I don't know why your aro isn't growing, but personally I don't think it is because of water parameters. I used to have a silver and never used those test kits or anything and he grew well until I gave him to my cousin. I just made regular water changes.
Make sure he's feeding well and not just on stapled prepared diets, because they're not that nutritious. Keeping him warm is also another thing to keep in mind. Also, make sure he isn't overcrowded in that pond.
