Please help, loosing too many fish!

Hey there, I've got a 130L tank using external pump, no protein skimmer. All the other normal stuff heater, 2 wave makers, lights. I've had the tank set up for 7 months after moving house and tank, everything has been fine up until the last 2 months, I had 5 fish die within 2-3 weeks and another one today about 4 weeks later a cardinal emporer. I have done regular tests on my water and they all come out good, I've taken water samples to my local fish shop for them to test and again results were fine, I have 3 corals in my tank which are all doing well In the water. I don't know whether something is killing them? They always die at the back of my tank behind the rock so I haven't managed to remove any of the dead bodies, could this of triggered the rest? About 2 months ago I added about 8 fish at the same time from a friend but it's been a mixture of them and 2 of my own fish that have died. I had 2 clown fish and added a 3rd that seems to be aggressive, but could that of killed all those fish? Any help would be much appreciated as within the next few weeks I'm getting a new tank and would like to solve the problem rather than just move it and loose more fish and money. Many thanks