diff --git a/src/hooks/__tests__/useOracle.test.ts b/src/hooks/__tests__/useOracle.test.ts
index 06a1d35..9ec3fc7 100644
--- a/src/hooks/__tests__/useOracle.test.ts
+++ b/src/hooks/__tests__/useOracle.test.ts
@@ -1,6 +1,7 @@
 import { renderHook, act, waitFor } from '@testing-library/react';
 import { vi, describe, it, expect, beforeEach, afterEach } from 'vitest';
-import { useOracleReading } from '../useOracle';
+import { useOracleReading, useAllOracleReadings, VISIBILITY_REFETCH_MIN_MS } from '../useOracle';
+import { fetchAllOracleReadings } from '@/lib/api';
 
 const mockFetchOracleReading = vi.fn();
 
@@ -220,4 +221,57 @@ describe('useOracleReading', () => {
     addSpy.mockRestore();
     removeSpy.mockRestore();
   });
+
+  // Issue #519: rapid tab switching must not trigger a fetch per
+  // visibilitychange -- only once the last fetch is older than
+  // VISIBILITY_REFETCH_MIN_MS.
+  it('throttles visibilitychange refetches to VISIBILITY_REFETCH_MIN_MS', async () => {
+    mockFetchOracleReading.mockResolvedValue({
+      key: 'weather-abuja',
+      dataType: 'weather',
+      value: '324000000',
+      confidence: 95,
+      timestamp: 1000,
+      source: 'mock',
+    });
+    let now = 1_000_000;
+    const nowSpy = vi.spyOn(Date, 'now').mockImplementation(() => now);
+
+    renderHook(() => useOracleReading('weather-abuja'));
+    await waitFor(() => expect(mockFetchOracleReading).toHaveBeenCalledTimes(1));
+
+    for (let i = 0; i < 10; i++) {
+      now += 100;
+      act(() => { document.dispatchEvent(new Event('visibilitychange')); });
+    }
+    expect(mockFetchOracleReading).toHaveBeenCalledTimes(1);
+
+    now += VISIBILITY_REFETCH_MIN_MS;
+    act(() => { document.dispatchEvent(new Event('visibilitychange')); });
+    await waitFor(() => expect(mockFetchOracleReading).toHaveBeenCalledTimes(2));
+
+    nowSpy.mockRestore();
+  });
+});
+
+describe('useAllOracleReadings', () => {
+  it('throttles visibilitychange refetches to VISIBILITY_REFETCH_MIN_MS', async () => {
+    const mockFetchAll = vi.mocked(fetchAllOracleReadings);
+    mockFetchAll.mockClear();
+    let now = 5_000_000;
+    const nowSpy = vi.spyOn(Date, 'now').mockImplementation(() => now);
+
+    renderHook(() => useAllOracleReadings());
+    await waitFor(() => expect(mockFetchAll).toHaveBeenCalledTimes(1));
+
+    now += 100;
+    act(() => { document.dispatchEvent(new Event('visibilitychange')); });
+    expect(mockFetchAll).toHaveBeenCalledTimes(1);
+
+    now += VISIBILITY_REFETCH_MIN_MS;
+    act(() => { document.dispatchEvent(new Event('visibilitychange')); });
+    await waitFor(() => expect(mockFetchAll).toHaveBeenCalledTimes(2));
+
+    nowSpy.mockRestore();
+  });
 });
diff --git a/src/hooks/useOracle.ts b/src/hooks/useOracle.ts
index fe95d97..de6caf1 100644
--- a/src/hooks/useOracle.ts
+++ b/src/hooks/useOracle.ts
@@ -5,6 +5,15 @@ import { fetchOracleReading, fetchAllOracleReadings } from '@/lib/api';
 import type { OracleReading } from '@/types';
 import { ORACLE_REFRESH_INTERVAL_MS } from '@/lib/constants';
 
+// A tab becoming visible only triggers a refetch if at least this long has
+// passed since the last fetch, so rapid tab switching can't flood the API
+// (#519). Interval polling remains the primary refresh mechanism.
+export const VISIBILITY_REFETCH_MIN_MS = ORACLE_REFRESH_INTERVAL_MS / 2;
+
+function isStale(lastFetchAt: number): boolean {
+  return Date.now() - lastFetchAt >= VISIBILITY_REFETCH_MIN_MS;
+}
+
 export function useOracleReading(key: string | null) {
   const [reading,  setReading]  = useState<OracleReading | null>(null);
   const [loading,  setLoading]  = useState(false);
@@ -12,10 +21,12 @@ export function useOracleReading(key: string | null) {
   const isFirstLoad = useRef(true);
   const currentKeyRef = useRef(key);
   const prevKeyRef = useRef(key);
+  const lastFetchAtRef = useRef(0);
 
   const load = useCallback(async () => {
     if (!key) return;
     currentKeyRef.current = key;
+    lastFetchAtRef.current = Date.now();
     const isFirst = isFirstLoad.current;
     if (isFirst) {
       setLoading(true);
@@ -56,7 +67,9 @@ export function useOracleReading(key: string | null) {
     if (!key) return;
     void load();
     const interval = setInterval(() => { if (!document.hidden) void load(); }, ORACLE_REFRESH_INTERVAL_MS);
-    const onVisible = () => { if (!document.hidden) void load(); };
+    const onVisible = () => {
+      if (!document.hidden && isStale(lastFetchAtRef.current)) void load();
+    };
     document.addEventListener('visibilitychange', onVisible);
     return () => {
       clearInterval(interval);
@@ -73,11 +86,13 @@ export function useAllOracleReadings() {
   const [error,    setError]    = useState<string | null>(null);
   const isFirstLoad = useRef(true);
   const refetchController = useRef<AbortController | null>(null);
+  const lastFetchAtRef = useRef(0);
 
   // Guards against a slower in-flight response overwriting a newer one's
   // state (#450) -- mirrors usePolicies'/useClaims' AbortController pattern,
   // the codebase's established fix for this class of stale-response race.
   const load = useCallback(async (signal: AbortSignal) => {
+    lastFetchAtRef.current = Date.now();
     const isFirst = isFirstLoad.current;
     if (isFirst) {
       setLoading(true);
@@ -104,7 +119,9 @@ export function useAllOracleReadings() {
     const interval = setInterval(() => {
       if (!document.hidden) void load(controller.signal);
     }, ORACLE_REFRESH_INTERVAL_MS);
-    const onVisible = () => { if (!document.hidden) void load(controller.signal); };
+    const onVisible = () => {
+      if (!document.hidden && isStale(lastFetchAtRef.current)) void load(controller.signal);
+    };
     document.addEventListener('visibilitychange', onVisible);
     return () => {
       controller.abort();