Do you fill your tank up fully?

In most cases, I would like to have it filled up to the very top but I have realized that my fishes loves that little open space on top, so I do leave an inch gap space for them to explore.

It depends on the kind of fishes that I have at the moment in my tank. If they are the type that loves to come afloat of the water, I will definitely leave a space for them but if not, I will have it filled fully.

I fill it up until the point where it is just below the filter water return. I like to do it this way to minimize the return of pushing the fish and shrimp around too much.

As someone who hates to see any marks on the side of the tank, I always tend to fill my tank up to almost full but not to the top. The reason I do not fill it right to the top is so that there is no ability the water can spill if I have to reach in to do something, I would hate to fill it to the top and then have to reach in for something and end up losing quite a bit of water just for the sake of that little bit extra I didn't fill.

To clarify, I fill may tank up “full,” but not all the way to the brim.

It's always better to leave a little space while filling up the tank and leaving just an inch level.