Water weighs 8.8 lbs per gallon. 40 gallons would be 352 lbs (not KG). A 40 gallon tank will probably have less than 40 gallons of water, but with rocks and filters and substrate and all a little extra weight is added in. You'll probably be closer to the 400-425 lb range. While heavy it is not an unreasonable load for your home to support if it is in decent shape. I think that it would weigh about as much as my ex-wife and she never fell through into the living room.
I wouldn't worry about a 40 gallon tank too much, a 400 gallon tank could be a problem though.
