Don't skip this NOTE: As before, we are assuming that you have a sort of "snapshot" of the tank, so we don't need to worry about fish moving around. But there is still a slight problem. What if a fish is "halfway" in one square and "halfway" in another? Like this:

It might seem to be a good idea to count fish that are "halfway or more" into the gridsquare, but actually it's NOT. The reason is that it introduces some subjectivity -- and worse yet, takes extra time, which could be better spent out having a social life...

We certainly don't want to count EVERY fish that is partway in the gridsquare, because that way we could overcount (for example, if we counted the adjacent gridsquare, any "in between" fish would get counted twice). And we don't want to count none of them, because then we'd be undercounting.

So, a compromise: we count only fish that touch the BOTTOM or RIGHT sides of the gridsquare. Even if they are just barely touching, almost entirely outside of the gridsquare. That way, no danger of under- or over-counting, less thinking and calculating, and more time outside the lab. Win-win.