diff --git a/frontend/src/lib/zen/aquarium.js b/frontend/src/lib/zen/aquarium.js
index e7c2eb4..11531d1 100644
--- a/frontend/src/lib/zen/aquarium.js
+++ b/frontend/src/lib/zen/aquarium.js
@@ -19,7 +19,8 @@ import { GLTFLoader } from 'three/addons/loaders/GLTFLoader.js';
 const MODEL_URL = '/models/ub-angelfish.glb';
 
 // UB is now a marine angelfish (Queen): ONE mesh, TWO materials (…_body, …_fins — fins
-// covers the caudal tail too), and a single trimmed Idle loop. Body renders OPAQUE +
+// covers the caudal tail too), and a single trimmed loop (Swim1_norm — a ~2.5s in-place
+// swim cycle, zero root drift, so it undulates without traveling). Body renders OPAQUE +
 // single-sided; fins default to OPAQUE alpha-tested (clean silhouette, no blend bleed —
 // the koi's lesson) but can go translucent. All live-tunable at /zen?debug=1.
 export const DEFAULTS = {
@@ -120,7 +121,7 @@ export async function createAquarium(canvas, initial = {}) {
   const mixer = new THREE.AnimationMixer(ub);
   const actions = {};
   for (const clip of gltf.animations) actions[clip.name] = mixer.clipAction(clip);
-  const baseClip = mixer.clipAction(gltf.animations[0]);   // the trimmed Idle loop
+  const baseClip = mixer.clipAction(gltf.animations[0]);   // the trimmed swim loop
   baseClip.play();
   const baseDuration = baseClip.getClip().duration || 1;
   mixer.timeScale = reduced ? 0.6 : 1;   // calmer when reduced-motion
diff --git a/frontend/static/models/ub-angelfish.glb b/frontend/static/models/ub-angelfish.glb
index 54ba103..68d302c 100644
Binary files a/frontend/static/models/ub-angelfish.glb and b/frontend/static/models/ub-angelfish.glb differ
diff --git a/tools/glb-split/trim-clip.mjs b/tools/glb-split/trim-clip.mjs
new file mode 100644
index 0000000..8e39c55
--- /dev/null
+++ b/tools/glb-split/trim-clip.mjs
@@ -0,0 +1,56 @@
+// Extract ONE clip [tStart,tEnd] (seconds) from a GLB's baked Take, rebased to start at 0,
+// and report the root joint's net translation over the clip (so we know if a swim cycle
+// drifts the fish through space vs. swims in place). Usage:
+//   node trim-clip.mjs in.glb out.glb 3.667 6.167
+import { NodeIO } from '@gltf-transform/core';
+import { ALL_EXTENSIONS } from '@gltf-transform/extensions';
+import { prune } from '@gltf-transform/functions';
+
+const [, , inPath, outPath, s0, s1] = process.argv;
+const tStart = parseFloat(s0), tEnd = parseFloat(s1), EPS = 1e-4;
+const io = new NodeIO().registerExtensions(ALL_EXTENSIONS);
+const doc = await io.read(inPath);
+const root = doc.getRoot();
+
+// Pass 1: per unique input, find the kept index window + rebase times to 0.
+const win = new Map();  // input -> {a,b}
+for (const anim of root.listAnimations())
+  for (const sm of anim.listSamplers()) {
+    const input = sm.getInput();
+    if (win.has(input)) continue;
+    const t = input.getArray();
+    let a = 0; while (a < t.length - 1 && t[a] < tStart - EPS) a++;
+    let b = t.length - 1; while (b > a && t[b] > tEnd + EPS) b--;
+    win.set(input, { a, b });
+    const base = t[a];
+    input.setArray(t.slice(a, b + 1).map((x) => x - base));
+  }
+// Pass 2: slice each output to its input's window.
+for (const anim of root.listAnimations())
+  for (const sm of anim.listSamplers()) {
+    const { a, b } = win.get(sm.getInput());
+    const out = sm.getOutput();
+    const c = out.getElementSize();
+    out.setArray(out.getArray().slice(a * c, (b + 1) * c));
+  }
+
+// Report root-joint translation drift over the clip (first joint's translation track).
+const skin = root.listSkins()[0];
+const rootJoint = skin?.listJoints()[0];
+let drift = 'n/a';
+if (rootJoint) {
+  for (const anim of root.listAnimations())
+    for (const ch of anim.listChannels()) {
+      if (ch.getTargetNode() === rootJoint && ch.getTargetPath() === 'translation') {
+        const o = ch.getSampler().getOutput().getArray();
+        const n = o.length / 3;
+        const dx = o[(n - 1) * 3] - o[0], dy = o[(n - 1) * 3 + 1] - o[1], dz = o[(n - 1) * 3 + 2] - o[2];
+        drift = `Δ[${dx.toFixed(4)}, ${dy.toFixed(4)}, ${dz.toFixed(4)}] (model units)`;
+      }
+    }
+}
+await doc.transform(prune());
+await io.write(outPath, doc);
+const a = root.listAnimations()[0];
+let tmax = 0; for (const ch of a.listChannels()) { const arr = ch.getSampler().getInput().getArray(); tmax = Math.max(tmax, arr[arr.length - 1]); }
+console.log(`wrote ${outPath}  | clip duration ${tmax.toFixed(2)}s  | root drift ${drift}`);