What is happening with my aquarium?

Hi, I have 3 aquariums, and I am currently having a major problem with my first. It has been established for over a year, and I have had no problems or deaths. I moved some of my larger tropical fish into my largest tank about 4 weeks ago, and gradually added some neon Tetra. I added 10 more neon Tetra a week past Wednesday. (The tank is large enough, and store sold in batches of 10 for �9.00). The following day I noticed 1 dead Tetra. The next day I found 3 dead Tetra and my Siamese fighting fish was also dead. We are now 10 days on, and I have lost fish every day. To date I have lost 20 Tetra, 1 young Platy and my Siamese. I currently have 2 Tetra in my breeder tank inside the main tank. They are there for their own protection as they look weak and next to go. My water has been cloudy for 4 days now. I have done various small water changes, and also bought a new filter. I rinsed one set of new cartridges in aquarium water and kept one set in from the old filter. I have tested my water, and also taken it to the shop to be tested. My PH was slightly off on one test, but is now fine, as is all others. I am not overfeeding, and have barely fed for the last couple of days. I thought perhaps the last batch of Tetra I added may have been diseased, but the pet store have had no other notification of a problem. Can you please help. I don't want to transfer my surviving fish into the other tanks, in case they all now have a disease. The only other thing that is sometimes slightly off on some tests is KH. Sometimes fine, and other times just slightly off. None of the fish have any visible sign of disease. I always add my water conditioners and stress zyme. I also add a small handfull of aquarium salt weekly. Please help.