Once a fish becomes tame, it appears there's nothing to make/keep it happy. The grooming station doesn't(?) work underwater and IIRC correctly from the Twitch stream, the fish feeder is meant as a replacement for it, but it doesn't affect happiness. This in practice means that as soon as a fish becomes tame, it will not reproduce, which means using the fish feeder for breeding will actually prevent breeding.
Steps to Reproduce
- Make a fish tame. - Try to get it to reproduce.
This issue has been fixed in either a future version or updated in a previous version.