It’s truly not that big of an effort. It’s not like you have to redo a 1000 gallon tank. I’ve had trouble with my 29 before, because of too deep of a substrate I might add, and I had it completely torn down, bare bottom, no decor, and then back up and running in less than two hours on my own. It may seem like a big task, but it’s really not. To say that you see other people with this deep substrate all the time is like that old saying, if everyone were jumping off a bridge, would you do it too?”
Your arowana was a much hardier fish than discus are. It will be fine as long as nitrates aren’t sky high, and even then it will probably be ok for quite a while. Discus start to have issues anywhere between 10-20 ppm of nitrates. It’s a whole different ball game and you can’t compare the two.
In addition, the anaerobic bacteria that creates the toxic gas thrives in deep undisturbed substrate. You will not know it’s there until it’s released and then it poisons the water killing all of the fish in a very short time. Even after that happens, you will probably still wonder what happened to your fish because you will not have seen it happen.
I’ve given you a very clear step by step set of instructions that you can use to easily and effectively get your tank setup to handle discus. Right now you’re sounding exactly like the people over on Simply Discus that don’t believe all of the experienced discus keepers when tell them what kind of setup and maintenance these fish require. I’ve seen the end result of that many times.
If this small effort to get your tank ready is too much for you to go through then you are probably better off skipping discus altogether because they will require a lot more effort than that over their lifetime.