I'm unsure if this is within the scope of the mod but I think it'd be great if custom fish could be tracked via an entity rather than an item. The entry could be completed by killing it, looking at it when in a certain range, interacting with it, something like that. Certain fish from other mods don't drop themselves as items so I think it'd be neat if they were still able to be tracked. Of course, this wouldn't need to replace the current system, but it would be nice to have as an option at least.

If not that, then support for bucket items would at least be a good middle ground. You could either modify removeRawTextInName or just add a name field. A name field might be better just for the sake of customization tbh, and also for mobs like Alex's Mobs' Catfish that have different bucket items for each variant. It could also be good to allow multiple entries in the item field for a similar reason.

Something I had looked at was adding both the item and the entity model of each fish to display in the journal, which I still want to implement at some point. Of course, if the fish doesn't have an entity model, or if it doesn't have an item associated with it, it would still appear as an entry as long as it has one of the two.

So yeah, this feature is planned!