The best situation is to have only a single pair per 10g tank that way they don't get stressed, which could cause them to eat the eggs. Sometimes the male or female won't be allowed near the eggs by the other parent and it's best to remove the one that isn't being allowed near them. This doesn't happen often but it does happen. The purpose of a breeding tank is to allow you to produce the most fry that you can by being able to control the environment that the fish are in. Other tankmates will cause stress, may eat the eggs, etc. Granted this happens in the wild, but then again you may only end up with up to 5% of the eggs surviving to adulthood, and sometimes that's MANY times more than actually survive. Some fish can produce tens of thousands of eggs and they only have 3-4 survive.