I have had numerous Guppies and four Corydoras aeneus swimming around in a 10 gallon aquarium that was primarily set up for my six Blue Tailed Newts. I had been hoping to breed the newts but to date I have not had any success in getting them to spawn. There has got to be a trigger but I have not figured it out. On the other hand, the Guppies breed like...well, like guppies and they keep doing it. The green Corys also bred frequently. I usually do a weekly water change and they usually breed right after that but not weekly. I'd say they have bred about every 3rd week on average. I think I have two males and two females. I used to have one other but it died for some unknown reason.
I have had this tank set up this way for about 2 years, or so I am guessing. About 6 months ago, I added something it did not have, an under-gravel filter. I know that many frown on these nowadays but I like them because they make the tank easier to maintain in my opinion. I remove the upright air tube and then insert a siphon hose and siphon the water from under the gravel when I do weekly tank maintenance. Today though was time to strip the tank down and give it, the gravel and all the accessories a thorough good cleaning. I do that about twice a year with my small tanks. Once I siphoned out most of the water, I scooped out most of the gravel, then lifted out the under gravel filters. Much to my surprise there were several fish under the filters. Even more to my surprise was the fact that about 6 of them were Green Aeneus catfish! The rest were either guppies and maybe one or two very tiny baby catfish. Most surprising of all was that one of the green cory, that I found under the filters, was about 1/3 to 1/2 grown.
I had always figured that my Corydoras aeneus were sterile since none of the eggs they laid ever seemed to hatch. What was happening though seemed to be that the larger guppies of those in the tank were eating them, or at least eating part of each egg almost as soon as each was laid and the remains stayed stuck to the glass. The cory's stick the eggs to the side of the aquarium glass, usually right in the stream of water coming from the power filter. I had mistakenly believed that the eggs were always bad but now see that was not the case. My guess is that when the guppies pecked away at the eggs, some good ones fell to the gravel and were sucked through the under-gravel filters where they hatched and where the babies, at least some of them apparently found enough to eat. I am truly amazed they could live under the filters as the water I siphon from under them when cleaning the tank is always dark brown to almost black. That is a result of anything organic that gets sucked under the filters and then decomposes there.
Well they lived and I now have about a half dozen of them swimming around in the tank. I removed all the guppies except some very tiny babies and am waiting to see what happens next time the catfish breed. Hopefully all the eggs will be fertile and I will get to see them develop and hatch.
All the best,