how do you tell yourself if enough is enough?
I'm going to just respond to your sentence.
Your post relates to impulse control. It's a very normal feeling that is fundamental to being human. (Let's skip the psychology of it.)
While most people have to train themselves out of it to a great extent while they are teens, it's common for most if not all people to retain some degree of it well into adulthood about certain things. Indeed, many people celebrate it as being "spontaneous", although later the same people may call it reckless or selfish. (I'm not judging. I've made a buttload of spontaneous but highly regrettable choices.)
To forestall it, I try to imagine the result of some of my decisions before I make them. So, in this case, I imagine what would happen if I acquired the fish and all of the fish I had grew to their expected size. I foresee whether or not that is going to be a problem, and I act accordingly.
I found that once I did that a couple times for a particular type of decision (like what car to buy, or what appliance to buy, what time to leave for the airport), I became more thoughtful about it the next time around. It's like my brain remembers the process I used before and it becomes a habit.
If you don't have a habit of thinking about the fish in that way, it's a lot easier to skip that part of the process. And in that case, of course, a large tank with five 2" inch pacu is absolutely crying out for more fish.
