Id all the way and yes it will get bigger than 18 inches whoever told you 18 inches doesn't know. Try 39 inches. It will spook easy and crash into the sides of the tank. IMO these guys belong in ponds only.
Thanks all i went to pick the flagtale up and ended up with the id shark too. He should be ok in my tank for a while but i only thought 18inch at the time and he would have been good for life ohh well i best do some reading up on the little fella
I had an ID before when I was a kid and he constantly smashed into the aquarium, now I have an indoor pond and have a 2 foot ID shark and I have to say, it is so calm in the pond, no sudden movements, no nothing, he drifts and ever so smoothly and calmly cruises around the pond. He's been with me two weeks and comes even and investigates my hand (provided I don't make any jerky movements).