Do I need an app for tabs on the product page?

No. Avatar already has this feature.
To activate a new tab you will have to:

1. Sign into your Shopify admin
2. Go to 'Online Store' > 'Theme'
3. Click Customize

app1

4. Go to the default product page
5. Select Products Tabs
6. Check “Show First" (or second) additional tab and write something nice

app2