do you mean you want to open the top of the bracing up...ie you have largish tank with smallish opening on top and you wish to make the openings bigger?
fairly simple if that is the case, though i would be pretty nervous about dropping the needed tool in the tank...this should be done with a router...drop the water level a couple inches, place large plastic sheet on the water to protect it from shavings/ dust etc...set router guide and run around the opening to the desired size
but you would need to take a couple pics of the tank for us to see what your dealing with...removing part or all the brace could certainly have serious structural impacts on the tank...hitting it with a hammer will definitely be a very bad idea, it could cause catastrophic failure of the brace/ tank...