If the bedding in a bin gets too wet it cannot breathe and becomes anaerobic. Therefore the worms can't get the oxygen needed to survive. Your worms in the aquarium gravel had oxygen and food(uneaten fish food and fish poop). I'm surprised that you have only found two worms. Maybe the fish ate the babies.

I have wondered about little ones. I thought that either A ( like you said ) the fish ate the babies which is most probable. or B. the aquarium isn't the prime place for one worm to find another willing worm. ( rather big tank and only two worms is hardly a good test case. ) But I was still amazed that they lived that long!... oh when they were in the tank.... it was never cleaner! not before and not after! so I do believe they were eating the left over fish food.

WOW!
You know I don't think I would have expected them to survive either. I have heard of people putting worms into the grow beds of Aquaponic systems but those are usually not submerged 24/7 and they are well aerated.

I expect they were eating all sorts of organic stuff and helping keep things clean.

Very, very interesting. I thank you much shawnrd for sharing that experience with us, I think it could turn into a useful thing to know. Just curious, why didn't you put the worms back into the aquarium after cleaning?

Here is a link that might be useful: My Bin

I didn't put them back into the tank mostly because at the time I was trying to see if they would work as fish food. And that experiment obviously failed horribly as they were alive and kicking and staying plenty far enough away from the fish to become food. So I just put them back into the worm bin when I found them. I've recently given the tank away to a friend so I can't try it again. I guess the only thing I can say if you want to try it yourself is that I was EXTREMELY hands off on the workings of the aquarium only changed half the water every six months (well water) and only cleaned it about once a year. It was as I said fresh water and about the only thing I added besides fish food was a little salt crystal stuff that the pet store guy suggested I add (still confused about that adding salt to a fresh water aquarium thing.. but it worked). The fish seemed happy ( had many generations of Mollys ) and obviously the worms were happy.