Never add too many or too few fish at once, never add any fish unless your aquarium has been running perfectly and it has been at least two weeks since a fish has died or been added. When you first introduce fish to your aquarium, they are quite stressed and their immune systems are quite compromised, so the balance of pathogens and antigens need time to stabilise. There are also various biological balances that need to occur when you add a new fish to the aquarium. So, what we want to do is add a small amount of fish, then wait two weeks or so, then get another small amount of fish, then wait two weeks or so for stability in the aquarium in regards to antigens, pathogens and biological processes. We don’t want to just go dumping a whole lot of fish on one day or dump a whole lot of fish over the span of a few days or even dump a whole lot of fish over the span of the coming weeks. The system just can’t handle it and it will end up crashing - you may end up getting a disease or very high levels of nutrients, killing your fish.

For a marine tank, we have to add a couple of fish and wait for four weeks, then we add another couple of fish, then wait for another four weeks. When you introduce a fish, it has various pathogens which are controlled by various antigens which are a part of the fish’s immune system. So, when the fish is stressed, like when you catch them and throw him in a bag, their immune system goes down, and isn`t producing as many antigens or as much body slime. Therefore, the pathogens, which are normally controlled by the antigens, start to go on a rampage, going into to the environment looking for a new host as a single-celled organism. They then reproduce on a hard surface and they come back as a dinoflagellate, which is a single-cell organism with a tail looking for a new host. They swim around and then try to attack your fish. So, depending on the species of pathogens, it could be within the first week, the second week, or the third week of their life cycle.

With marine fish, it is very important that we wait the full four weeks, because otherwise the new fish often get sick due to parasites released from the last set of fish. Since their immune system is low and their body slime is not intact, their antigen level is not intact. With the freshwater fish, the time between fish introductions doesn`t seem to matter so much - 2 weeks seem to be ok. So, exactly the same phenomena are occurring in the freshwater as with saltwater, but we just don’t seem to have as many problems. Koi, for example, are normally set for four weeks, as well. With freshwater, this is not a big drama. In saltwater, this is a huge drama.

Now, the next thing is I say is: Not too many. With too many fish, there is more waste than the bacteria in the system are able to break down. On the other hand, I also say don`t put in too few fish. This would often be a problem in an aquarium that has been established for some time and they have well-established territories. Then when you add two new fish the focus of all the other fish is on these two new fish. When a new one swims into another`s territory it will get pushed out! As if the fish is saying “This is mine!" and swims out, and another says, "Get out of here! This is mine!" He ping-pongs all around the tank and can’t find anywhere he is comfortable because all the nice homes have been taken. If the fish is not comfortable its stress levels go up and the more stressed it is the more likely it is to get picked on. It`s just like being put into a new neighborhood and being unable to find anywhere to live. Naturally, the fish will become quite stressed, and thus much more likely to get sick. The more fish go in at once, the safer it is for them getting along. You just have to ensure that the amount you put in does not disrupt the bio-load.

As per.com Paul Talbot
Taken from www.cichlidhub.com