diff --git a/oephys2nix/tonix.py b/oephys2nix/tonix.py
index 60c41da..bc13b8b 100644
--- a/oephys2nix/tonix.py
+++ b/oephys2nix/tonix.py
@@ -7,6 +7,7 @@ import numpy as np
 import rlxnix as rlx
 from IPython import embed
 from neo.io import OpenEphysBinaryIO
+from scipy.signal import butter, sosfiltfilt
 
 from oephys2nix.logging import setup_logging
 from oephys2nix.metadata import create_dict_from_section, create_metadata_from_dict
@@ -68,15 +69,6 @@ class RawToNix:
     def append_fish_lines(self) -> None:
         """Append fish lines from open-ephys."""
         efishs = ["ttl-line", "global-eod", "stimulus", "local-eod", "sinus"]
-
-        efish_types = [
-            "open-ephys.data.sampled",
-            "open-ephys.data.sampled",
-            "open-ephys.data.sampled",
-            "open-ephys.data.sampled",
-            "open-ephys.data.sampled",
-        ]
-
         efish_group = self.block.create_group("efish", "open-ephys.sampled")
 
         efish_neo_data = self._load_neo_object(["Data_ADC", "acquisition_board_ADC"])
@@ -86,7 +78,7 @@ class RawToNix:
             efish_neo_data_array = efish_neo_data[:, i]
             data_array = self.block.create_data_array(
                 f"{efishs[i]}",
-                f"{efish_types[i]}",
+                "open-ephys.data.sampled",
                 data=efish_neo_data_array.magnitude.flatten(),
                 label="voltage",
                 unit="V",
@@ -178,6 +170,72 @@ class RawToNix:
         nix_data_array.append_sampled_dimension(1, label="channel")
         gr.data_arrays.append(nix_data_array)
 
+        preprocessed_data = self.preprocess_raw(nix_data_array)
+
+        preprocessed_data.append_sampled_dimension(
+            1 / raw_neo_data.sampling_rate.magnitude, label="time", unit="s"
+        )
+        preprocessed_data.append_sampled_dimension(1, label="channel")
+        gr.data_arrays.append(nix_data_array)
+
+    def preprocess_raw(
+        self,
+        data: nixio.DataArray,
+        batch_seconds: float = 20.0,
+        overlap_seconds: float = 1,
+        band_hz: tuple[float] = (300.0, 6000.0),
+        filter_order: int = 3,
+    ) -> None:
+
+        fs = int(1 / data.dimensions[0].sampling_interval)
+        n_samples = data.shape[0]
+
+        batch_samples = int(round(batch_seconds * fs))
+        overlap_samples = int(round(overlap_seconds * fs))
+
+        low_hz, high_zh = band_hz
+        if not 0 < low_hz < high_zh < fs / 2:
+            raise ValueError("Band must lie striclty between 0 and Nyquist.")
+
+        sos = butter(
+            filter_order,
+            band_hz,
+            btype="bandpass",
+            fs=fs,
+            output="sos",
+        )
+        nix_data_array = self.block.create_data_array(
+            name="preprocessed-data",
+            array_type="open-ephys.data.sampled",
+            dtype=nixio.DataType.Float,
+            unit="uV",
+            shape=data.shape,
+        )
+
+        for start in range(0, n_samples, batch_samples):
+            stop = min(start + batch_samples, n_samples)
+
+            # Read extra samples on both sides of the output batch.
+            read_start = max(0, start - overlap_samples)
+            read_stop = min(n_samples, stop + overlap_samples)
+
+            block = data[read_start:read_stop].astype(np.float32)
+
+            ref = np.median(block, axis=1, keepdims=True)
+            x = block - ref
+
+            # Zero-phase filtering along time, independently per channel.
+            filtered = sosfiltfilt(sos, x, axis=0)
+
+            # Discard the overlap; retain only this batch's central samples.
+            keep_start = start - read_start
+            keep_stop = keep_start + (stop - start)
+
+            nix_data_array[start:stop] = filtered[keep_start:keep_stop]
+
+            log.debug(f"Processed {stop:,} / {n_samples:,} samples")
+        return nix_data_array
+
     def close(self) -> None:
         """Close all nix files."""
         self.nix_file.close()