It's patched out now, but if you still have a 1.6.8 launcher saved or are willing to downgrade, it's a possibility.
The way it worked is due to a few mechanics, one faulty (the one now patched).
First, if a fish pond produced roe (or really anything for that matter), it won't replace that with another drop the next days unless something is chosen. This means that if you don't harvest a fish pond, it'll never just be empty the day after. Second, the faulty mechanic was that the animal cracker would double anything in its basket in the morning, without checking that the content it was doubling was new.
That means, if you were to strategically place the fish pond on tiles that didn't produce new product (the way that fish pond loot is selected is like many other things, a seeded overlay that shifts per day in simplest terms), the cracker would double it. And you could keep doubling until 999.
The third mechanic is just a simple one, if you have an almost full inventory and collect, you can partially collect from certain machines and fish ponds. That means you could fill up your inventory with each slot having some junk item, and one slot with 500 roe, then when you collect the roe, you only get 500 instead of 999, BUT, the fish pond would be back to 999 the next day (the remaining 500 items in the fish pond doubling to 999 overnight).