We have new-ish hardwood floors. Our boards are prefinished, not finished after installation, so while water spills won't hurt the boards at all, I am slightly concerned about water seeping between them. I have had no major spills onto the floor yet, so I can't say if this concern is valid or not, but I imagine it will happen one day and I'll find out then.
In the mean time, even though small spills and spots won't actually hurt the floor, they're always pretty obvious in the light and a pain to keep cleaning up all the time.
So I've put down a section of
this waterproof slipproof material in front of my tank, extending 2.5' front to back, and off to both sides of the tank by 1.5'. It's very thin, and when placed on hardwood, will not move at all, and also leaves no marks. I covered it with a section of
reasonably attractive commercial carpet runner. This stuff is easy to clean, doesn't get moldy when wetted, and is
not waterproof, so small water spills largely go through and are caught by the layer below. Very easy to keep this setup looking nice, even if you're in and out of your tank a lot.