Ever since the Aquatic Update, every major update had made any newer fish unstackable with any older fish from before updating. The current fish will stack up until it updates again and then it won't stack any further. It's rather bothersome. Please make sure you fix it, for real this time. It has effected my realm multiple times.
Linked issues
Comments
GoldenHelmet
Auldrick
You haven't said what kind of fish you're having the issue with (cod, salmon, pufferfish, clownfish, tropical fish; raw or cooked in the case of cod/salmon). GoldenHelmet is assuming you mean cooked cod/salmon, which would make this a duplicate of MCPE-54617, and I agree that that's probably what you meant since other people had that problem. I'm therefore resolving this as a duplicate of MCPE-54617. If you meant some other kind of fish, or have some other reason to think this is an unrelated issue, feel free to comment with more information and we'll look at reopening this report.
Assuming this is, in fact, the same bug, the stacking problem should be the only symptom. You should be able to eat all the fish, use it to heal a wolf/dog, or destroy it if you prefer. Once you eliminate the pre-1.12 stacks from your inventory and storage, the problem should go away.
migrated
There was a furnace bug in 1.12 that caused items smelted in that version to be unstackable with items smelted in other versions. It was fixed in 1.13 so that furnaces no longer produce bugged items. Unfortunately, however, it is not possible for a bug fix to check and edit all of the item that already exist in your worlds. Please seem MCPE-54617 for more information.