rescue from back of nano

You could probably put a fine mesh across the entrance. This would allow the water to pass through but block the fish. Check out a home supply store for window screening--should offer some in plastic that will be safe to use.
 
Or get plastic grid. It's pretty tough stuff and you can get it at Walmart in the craft section for $.26 a 12x8 sheet
 
AquariaCentral.com