diff --git a/src/stack_to_chunk/main.py b/src/stack_to_chunk/main.py
index 741d9c2..bfa5369 100644
--- a/src/stack_to_chunk/main.py
+++ b/src/stack_to_chunk/main.py
@@ -213,7 +213,7 @@ def _create_zarr_group(self, array_spec: ArraySpec) -> None:
 
     @property
     def _full_res_array(self) -> zarr.Array:
-        if "0" not in self._group:
+        if 0 not in self.levels:
             msg = (
                 "Full resolution dataset not present. "
                 "Run `create_initial_datset()` first."
@@ -229,7 +229,7 @@ def levels(self) -> list[int]:
         Level 0 corresponds to full resolution data, and level ``i`` to
         data downsampled by a factor of ``2**i``.
         """
-        return sorted(int(k) for k in self._group)
+        return sorted(int(k) for k in self._group.array_keys())
 
     @property
     def chunk_size_z(self) -> int:
@@ -271,12 +271,6 @@ def add_full_res_data(
             z-index at which this stack of input data starts. Can be useful to write
             multiple slabs in parallel using a compute cluster where the job wants
             to be split into many small individual Python processes.
-
-        Notes
-        -----
-        Make sure create_initial_dataset has been run first to set up the
-        zarr dataset.
-
         """
         assert data.ndim == 3, "Input array is not 3-dimensional"
         if start_z_idx % self.chunk_size_z != 0:
@@ -365,18 +359,17 @@ def add_downsample_level(
             msg = "level must be an integer >= 1"
             raise ValueError(msg)
 
-        level_str = str(int(level))
-        if level_str in self._group:
-            msg = f"Level {level_str} already found in zarr group"
+        if level in self.levels:
+            msg = f"Level {level} already found in zarr group"
             raise RuntimeError(msg)
 
-        if (level_minus_one := str(int(level) - 1)) not in self._group:
-            msg = f"Level below (level={level_minus_one}) not present in group."
+        if level - 1 not in self.levels:
+            msg = f"Level below (level={level - 1}) not present in group."
             raise RuntimeError(
                 msg,
             )
 
-        source_arr: zarr.Array = self._group[level_minus_one]
+        source_arr: zarr.Array = self._group[str(level - 1)]
         source_chunk_shape = source_arr.chunks
 
         new_shape = tuple(math.ceil(i / 2) for i in source_arr.shape)
@@ -389,7 +382,7 @@ def add_downsample_level(
         )
 
         sink_arr = self._group.create_array(
-            name=level_str,
+            name=str(level),
             shape=new_shape,
             shards=new_shard_shape,
             chunks=source_arr.chunks,
@@ -413,15 +406,15 @@ def add_downsample_level(
             ]
         ] = [
             (
-                self._path / str(level_minus_one),
-                self._path / level_str,
+                self._path / str(level - 1),
+                self._path / str(level),
                 idxs,
                 downsample_func,
             )
             for idxs in block_indices
         ]
 
-        logger.info(f"Starting downsampling from level {level_minus_one} > {level}...")
+        logger.info(f"Starting downsampling from level {level - 1} > {level}...")
         blosc_use_threads = blosc.use_threads
         blosc.use_threads = 0
 
@@ -431,7 +424,7 @@ def add_downsample_level(
 
         self._add_level_metadata(level)
         blosc.use_threads = blosc_use_threads
-        logger.info(f"Finished downsampling from level {level_minus_one} > {level}")
+        logger.info(f"Finished downsampling from level {level - 1} > {level}")
 
     def _add_level_metadata(self, level: int = 0) -> None:
         """