A log for freshwater and saltwater tanks.
Email support@treamz.me. Say which iPhone and which iOS version, and what you did just before the problem — that is usually enough to find it.
Settings → Export. CSV for a spreadsheet, JSON for a complete copy. It is free, it needs no account, and it includes everything. You do not have to ask anyone for it, and you do not need this app to read it afterwards.
No. Signed out the app is complete and your data stays on the device. An account exists only so your tanks reach your other devices.
Settings → Account → Delete account. That removes the account and the data held on the server. The copy on this device is left alone; deleting the app removes that.