I've had a brief look at the ECG samples available at Physionet. Are you using this set? If so, each sample contains the raw ECG and a filtered version. Presumably the raw samples have already been low-passed
I'm studying Arduino to make a ECG platform
What exactly are you trying to implement on the MEGA - what do you mean by a "platform"? If all you're going to do is some offline processing of samples from the Physionet database (e.g. filtering, beat detection), the MEGA will suffice. But if you want to do real-time collection, analysis and display of ECGs, the MEGA would not be my first choice 
Pete