you can change the water down to the gravel. I have done it many, many times in my tanks. The fish will be stressed for the few seconds it takes to get water back in. I'd suggest changing out 95% a few times. There are days in my planted tank where i change the water repeatedly 3-4 flushes at 80% to clear debris from dusty old flourite after a rescape. I assure you I have never lost a fish doing this. Likewise, to beat out ich in a angelfish tank with breeding parents, i drained down to flopping levels every day for ten days for water changes. Again, didn't lose a single fish (well, lost some fry, but that was from sucking them up, not from suffocation or stress. The loss was collateral to me, since I knew the parents would breed again, and was more concerned about saving them and the other adult fish in the tank, including my prized BGK and tetras, from ich).
Secondly, unless there was creamer in the coffee (even still, depending on the amount of coffee spilled, and the volume of the tank) I wouldn't panic. Coffee is probably not that bad for your fish, unless they are something very needing of very hard water (african rift lakers). Many fish, in fact, would likely be inspired to breed if we added a small amount of used coffee grounds to the water in a cheesecloth....asian countries are famous for using almond and tea leaves to create "blackwater" for bettas to breed in. South American countries.......
Tea leaves, or coffee leaves.
Softens and acidifies the water, and adds tannins.
Even the creamer (unless artificial, never know what is in that stuff....how can a cup of liquid creamer not go bad sitting out in the open on a shelf for a year???) would likely not kill them unless we are talking about a LOT of creamer or a very small tank. It is just protein and fat and lactose when you break it down...not pleasant, but likely not deadly either. Fish can eat pretty amazing stuff. But this is something I certainly would be a bit more concerned about for the bacteria/fungus factor more than anything. If you had cream, do BIG water changes (the flopping kind) to get it out. That is what I would do anyway....but to each his own. I am pretty confident (i love my python....I can drain to floppage, and start the fill as soon as the floppage starts), and like I said, once I got over the initial "OHGODANYTHINGIDOMORETHAN10%WILLKILLTHEM" phase, I got pretty bold, and ironically, they like the bigguns (I consider a big H2O chage more than 70%. I rarely ever change less than 50%).
Finally, i agree with zags that activated carbon in the filter is probably good, just for the caffiene soakage, and definitely for dairy soakage.
Good Luck!