The past two times my fish have died the drops haven't appeared in my special tab. I checked the aquarium tab too just to be sure they didn't end up there some how.
I only have one page on it. I only have like 10 items in it.
Just because there's only a row or two of icons (or less) doesn't mean you don't have multiple pages with that tab. Special items are only partially-stacking; they'll show one icon and a number indicating how many you have, but each individual item is still taking up a spot. So if you have your page limit set to 600 items/page, you have 500 Philosophers Caches, 100 Emerald Caches, 50 Ruby ones, and some random fish drops, your page is full after just the original and emerald caches, so the rubies and fish drops will be on page two.
I didn't know that. But I did check there isn't a "next page" arrow or anything at the bottom. All I have in there is a fishing rod, some old valentine photos and a gender potion.