I think the last thing he needs is more, he will already need a much larger tank or a pond as it is with just the hifin and single ID, both of which are capable of growing to well over 24", and ID's do fine kept singly anyway. To the OP, you are eventually going to need a tank of at least 240-360 gallons, and the ID will need to be separated from the other fish (it will likely eventually eat everything else that is currently in the tank except for possibly the hifin, but the hifin and ID will also need to be separated anyway due to their different water requirements.
