The formula from the internet is to convert a displacement to acceleration. You have a velocity so you take the derivative of the velocity and you wrote a correct equation for acceleration.
The amplitude for the acceleration is A*w = 0.025*3.1416 = 0.07854 m/s^2. Don't evaluate the sin(w*t) term, which you got wrong because you didn't use radians, but you only want the amplitude anyway.
I will reply later after I find a reference for adding the fluid to the tank. That question has been asked before so you can try searching for it also.
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