Open Fish Fathom and choose More → Settings → Delete Account. When asked, sign in using the verification code sent to your account email, then return to Settings and confirm deletion.
The app confirms when the account has been deleted. Opening this page or an old email link does not delete an account or send a confirmation email.
Deleting your account does not cancel an App Store or Google Play subscription. Cancel your subscription in the store where you purchased it before deleting your account if you want to stop future billing.