Applying FFT giving all maxPeaks at 1

Question

I am trying to accomplish applying an FFT algorithm to set of filtered data. The filter I was asked to implement was a second order high pass IIR filter. I get the array of filtered data and then I pass to my FFT function. Everytime I do, I am just getting all max peaks at 1 .

I use it like this :

double[] fft = getFFT(iirFiltered);
        return fft;

And this is how my getFFT() function looks like

private double[] getFFT(double[] m1){

            double sum = 0;
            for (int i = 0; i < m1.length; i++) {
                sum += m1[i];

            }
            double mean = sum / m1.length;

            for (int i = 0; i < m1.length; i++) {
                m1[i] = m1[i] - mean;

            }

            int neft = 0;
            int count = 0;
            while (neft <= m1.length) {

                neft = (int) Math.pow(2, count);
                count++;
            }

            DSPLib.nFft(neft, m1);


            double[][] filtered = DSPLib.getFFT();

            double[] returnFiltered = new double[filtered.length];
            for(int i=1;i< returnFiltered.length;i++){
                //returnFiltered[i]= filtered[i][0]; // old one
                returnFiltered[i]= Math.sqrt(filtered[i][0]* filtered[i][0] + filtered[i][1]*filtered[i][1]); //new one

            }
            return returnFiltered;
        }

This is how I perform analysis :

private double[] performAnalysis (double[] rrIntervals) {
        ArrayList<Double> splineRRIntervals = SlplineInterpolator.interpolate(rrIntervals);


        ArrayList<Double> downSampledData = new ArrayList<>();

        for (float i = 0; i < splineRRIntervals.size(); i = i + 1000f / (float) sampleRate) {
            downSampledData.add(splineRRIntervals.get((int) i));
        }
        FileIO fileIO = new FileIO();
        //fileIO.writeDoubleArrayToFile("interpolation_down.txt", convertDoubles(downSampledData));

        double[] iirFiltered = new double[downSampledData.size()];
        for(int i=0;i< iirFiltered.length ;i++){
            if(i==0){
                iirFiltered[i] = IIR.iirHpf1ecg(downSampledData.get(i),0,0);

            }
            else if(i==1){
                iirFiltered[i] = IIR.iirHpf1ecg(downSampledData.get(i),downSampledData.get(i-1),0);

            }
            else{
                iirFiltered[i] = IIR.iirHpf1ecg(downSampledData.get(i),downSampledData.get(i-1),downSampledData.get(i-2));

            }


        }


        /*double[] iirFiltered = new double[splineRRIntervals.size()];
        for(int i=0;i< iirFiltered.length ;i++){
           if(i==0){
               iirFiltered[i] = IIR.iirHpf1ecg(splineRRIntervals.get(i),0,0);

           }
            else if(i==1){
               iirFiltered[i] = IIR.iirHpf1ecg(splineRRIntervals.get(i),splineRRIntervals.get(i-1),0);

           }
            else{
               iirFiltered[i] = IIR.iirHpf1ecg(splineRRIntervals.get(i),splineRRIntervals.get(i-1),splineRRIntervals.get(i-2));

           }

        }*/

        //added for IIR filter

        iirFilteredForPlot = new double[iirFiltered.length];
        for (int i = 0; i < iirFiltered.length; i++){
            iirFilteredForPlot[i] = iirFiltered[i];
        }

        //end for graph

        double[] fft = getFFT(iirFiltered);
        // logLFHF(); // added this new
        return fft;
    }

My output gives me a weird FFT graph and no matter whatever frequency I am on, maxPeak always comes at 1

Please guide me as I am new to signal processing.

PS : At a sampling rate of 4d, and heart rate of 240 bpm,I get 224 RR Interval values and 256 values for FFT output (this code somehow gives all real values only) , Max Magnitude: 19202.405718996466 , HF:: 5.35238130772106E7 , LF:: 6.235056793402985E7