What's new since build 17
Superseded. Build 18 went to open testing on 19 August and was replaced the next day by 0.5.1 (build 19), which is the current build — install that one from FishCoin's page on Google Play.
This is the whole build, and it is a repair rather than a feature.
In builds 15, 16 and 17 there was no way to sign in. Tapping "Start a challenge" or entering a tournament put up a dialog asking you to set up an account, with a button that took you to a screen where no account could be set up. There was no way through it. If you tried and gave up, nothing was wrong with what you did.
Now:
Worth reading once, because it is the only part with a consequence.
Signing in attaches your name to the account you have been using all along, so every catch you have already recorded stays yours. Nothing moves and nothing is lost.
The exception: if you have used the same Google account on another phone, FishCoin already has an account for you, and two accounts cannot be merged once both have catches. In that case you are signed in to the older account, and the catches recorded on this phone stay on the anonymous account they were made on. The app tells you when this happens rather than letting you discover it.
Recording a catch, measuring it and looking at your gallery all worked throughout, without an account, and are unchanged.
Unchanged, and still first: tape-measure a fish and enter the number — with the tail pinched to a point, as the prompt asks.