Hi all I have 40 litre tank which has been up and running for 4 months- we have only had fish in it for a month due to an initial problem with the quality of the water. Once all the water tests come back fine- We went to our local pet store and they recommended us getting a pair of Buenos aires tetras initially which we did however one of the fish ( we think female due to larger size and less colourful fins) has been very aggressive to the other tetra ( we think male- smaller and more colourful)from the start. He hides behind the filter constantly every time he attempts to come out from his hiding place she immediately chases him back in- he seems well in himself and eats regularly and when both fish feel under threat ie when doing the water changes they stick together for a while till the aggression starts again. We recently purchased 4 more fish on advice of the pet store ( the last fish we wil get due to size of tank) these are 2 cat-fish and I have forgot the name of the other two they are small orangey-yellow and have red tails they are settling in well but have had little effect on the aggressive fish who up to this point has only targeted the other tetra- we were hoping that getting the final four fish would take her dominance away. What more can we do?- we dont want to get rid of any fish if we can help it as so far no damage has been done we just have a fish which has been hiding for the last month.