diff --git a/reasoning_gym/arc/arc_1d_tasks.py b/reasoning_gym/arc/arc_1d_tasks.py
index c4de932a..b454220d 100644
--- a/reasoning_gym/arc/arc_1d_tasks.py
+++ b/reasoning_gym/arc/arc_1d_tasks.py
@@ -802,13 +802,13 @@ def task_duplicate_block_from_seeds(rng: Random, size: int) -> Optional[dict[str
 
 def task_fill_from_pixel(rng: Random, size: int) -> Optional[dict[str, list[int]]]:
     """Generate a task where a pixel fills in one direction until hitting another pixel."""
-    if size < 6:
+    if size < 8:
         return None
 
-    block_size = rng.randint(3, size - 3)
+    block_size = rng.randint(3, size - 5)
 
     # Position block with space for seed
-    block_pos = rng.randint(1, size - block_size - 1)
+    block_pos = rng.randint(2, size - block_size - 2)
 
     # Create input
     question = gen_field(size)
@@ -819,9 +819,9 @@ def task_fill_from_pixel(rng: Random, size: int) -> Optional[dict[str, list[int]
         question[block_pos + i] = block_color
 
     # Place seed pixel and determine fill direction
-    seed_color = rng.randint(1, 9)
-    while seed_color == block_color:
-        seed_color = rng.randint(1, 9)
+    seed_color = rng.randint(1, 8)
+    if seed_color >= block_color:
+        seed_color += 1
 
     is_left = rng.random() < 0.5
 
@@ -847,48 +847,51 @@ def task_fill_from_pixel(rng: Random, size: int) -> Optional[dict[str, list[int]
 
 def task_mark_size_two_blocks(rng: Random, size: int) -> Optional[dict[str, list[int]]]:
     """Generate a task where size-2 blocks are marked with surrounding pixels."""
-    blocks = []
-    pos = 0
+    if size < 8:
+        return None
 
-    # Generate blocks with minimum gap of 2
+    # Start with one size-2 block
+    blocks = [2]
+    pos = 4  # Space for first block (2) + gap (2)
+
+    # Generate more blocks
     while pos < size:
         if rng.random() < 0.4:
             block_size = rng.randint(1, 3)
-            # Check if we have space for block and potential markers
-            needed_space = block_size + (2 if block_size == 2 else 0)
-            if pos + needed_space < size:
-                blocks.append((pos, block_size))
-                pos += block_size + 2  # Minimum gap of 2
+            if pos + block_size <= size:
+                blocks.append(block_size)
+            pos += block_size + 2  # block + gap
+        else:
+            blocks.append(0)
+            pos += 1
 
-        pos += 1
+    # Shuffle block sizes
+    rng.shuffle(blocks)
 
-    if len(blocks) < 2:
-        return None
+    # Assign positions with proper gaps
+    block_positions = []
+    pos = 0
 
-    # Verify gaps between blocks (including markers)
-    valid = True
-    for i in range(len(blocks) - 1):
-        pos1, size1 = blocks[i]
-        pos2, _ = blocks[i + 1]
-        needed_gap = 3 if size1 == 2 else 2
-        if pos2 - (pos1 + size1) < needed_gap:
-            valid = False
-            break
-    if not valid:
-        return None
+    for block_size in blocks:
+        if block_size == 0:
+            pos += 1
+        else:
+            block_positions.append((pos, block_size))
+            pos += block_size + 2  # Move past block + gap
 
     # Create input with blocks
     question = gen_field(size)
-    for pos, block_size in blocks:
-        # Place block
+    for pos, block_size in block_positions:
+        block_color = rng.randint(1, 8)
+        if block_color >= 3:  # avoid marker color 3
+            block_color += 1
         for i in range(block_size):
-            question[pos + i] = 1
+            question[pos + i] = block_color
 
     # Create answer with markers
     answer = question.copy()
-    for pos, block_size in blocks:
+    for pos, block_size in block_positions:
         if block_size == 2:
-            # Add markers for size 2 blocks
             if pos > 0:
                 answer[pos - 1] = 3
             if pos + block_size < size:
@@ -935,7 +938,10 @@ def task_fill_until_collision(rng: Random, size: int) -> Optional[dict[str, list
 
     # Color random pixels
     for pos in positions:
-        question[pos] = rng.randint(1, 9)
+        c = rng.randint(1, 8)
+        if c >= 5:  # don't use side marker color 5
+            c += 1
+        question[pos] = c
 
     positions.sort()
 
diff --git a/tests/test_arc_1d.py b/tests/test_arc_1d.py
index 7f86db41..140916d6 100644
--- a/tests/test_arc_1d.py
+++ b/tests/test_arc_1d.py
@@ -1,3 +1,5 @@
+from random import Random
+
 import pytest
 
 from reasoning_gym.arc import Arc1DConfig, Arc1DDataset
@@ -125,3 +127,18 @@ def test_arc_1d_size_ranges(min_size: int, max_size: int):
         assert min_size <= len(entry["metadata"]["test_example"]["input"]) <= max_size
         assert min_size <= len(entry["metadata"]["test_example"]["output"]) <= max_size
         assert dataset.score_answer(entry["answer"], entry) == 1.0
+
+
+def test_arc_1d_generate_all_tasks():
+    config = Arc1DConfig(size=100, seed=17, min_size=8, max_size=10)
+    dataset = Arc1DDataset(config)
+    tasks = dataset.ARC_1D_TASKS
+    rng = Random(999)
+    for task_name, (generator_fn, args) in tasks.items():
+        for j in range(3):
+            for i in range(20):
+                x = generator_fn(rng=rng, size=10, **args)
+                if x is not None:
+                    break
+            assert i < 20
+            print(task_name, j, i, x)