The answer is why would it?
First off, dead weight (weight that never moves) is nothing like live weight (a person sitting on something for 10 minutes or an hour.) Wood, metal, all materials deform slightly when subject to weight. Over time they continue to deform depending on the type of material. Over time is the key word. Wood that will not collapse or fail after 1 hour, can fail under the same weight if it isn't moved for a week, a month or 6 months. That's because when live weight moves, the wood returns to it's original condition. Dead weight simply continues to cause deflection until it potentially fails.
Secondly, people who build houses don't construct them beyond what is needed. There is absolutely no reason to expect a person to place a cantilevered dead weight of 400 pounds on a counter with a thin support wall. The cost of building something to do that is lost profit because no one can see it or cares about it and most people will never use it. The counter is almost certainly built to hold what a reasonable person would place as a dead weight on the counter and perhaps 50% more. ("Perhaps.")
Keep in mind that the counter is a T on top of the wall, meaning that each side is structurally a cantilever. Regardless of how well the tank is centered, part of the tank is physically suspended in "thin air" with only the deflection of the counter top (the left and right sides of the T) keeping it off the ground. The angle supports under the counter top to the support wall are meant to support "reasonable" weights on the edge of the counter top, like a person standing on it for a moment, appliances, dishes, food, a small TV, etc.
Imagine placing a 10 gallon tank on the edge of the counter----not over the center of the support wall at all----on each side of the wall and leaving it there for 1 year. That's not a normal use. Over time, a 40 gallon (with edges being supported by the counter top), will be stressed from the counter top deflecting under the weight.
I suggest not.