I "adopted" my neighbours tank a little over a month ago, 30"x12"x18" - roughly a 20 gallon tall. He had about 24(!) cons in it, although most of them were juveniles from an original pair that had spawned. By this point, I had no idea which ones were the original pair, and it was a rescue job anyway - the tank had been in an apartment he'd rented out and the guy stayin there didn't know what he was doin - water conditions were terrible - don't know how they survived, let alone bred! So I got them home, stuck them in a holding tub 'til I got the tank set up (whole other story...). Suffice to say, they all survived the move and are happy and healthy. Was going to just keep a few to see if I could breed them, and trade in the rest at lfs, but I've been too busy to do so!
Guess what - they beat me to it!
Now I'm on my sixth spawning, around 3 or 4 pairs formed from the original 24 cons, and there's eggs in caves and fry in the gravel...!!!
And I still havn't got rid of the excess juveniles!
The moral of the story is, even if you only have two black Cons in your 29 gallon tank, rest assured, as long as you have a male and a female, it won't be long before you have a tank FULL of the little guys. Some plants, gravel, rocky caves and a flowerpot on it's side - Stocking problem SOLVED!
And don't put anything else in with them.....