did you cycle your tank in these 4 weeks? Also, you didn't mention nitrite level
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Hi and welcome to the forum.
What test kit are you using?
What are your readings for ammonia, nitrite, nitrate and PH?
What fish are in your tank and what fish have you lost?
Any signs of illness, gasping at the surface etc.?
Sorry to bombard you with questions but it may help discover what the problem is.
PH isn't too bad at 7.5 and that isn't likely to be the reason you are losing fish.
Nitrite and Ammonia should read 0 and more sensitive fish can't tolerate even low levels of either of these.
A lot of the fish you have mentioned prefer a more mature tank as changes in water quality can affect their health.
Stocking should be done slowly although some people prefer to stock straight away. I prefer to stock a few fish at a time and have had no problems with water quality using this method.
Fungal infections can be brought on by poor water quality so it would be worth doing a water change of about 30%.
Try using a medication such as Interpet No.9 which treats a range of bacterial infections and should help your fish. You will need to remove any carbon out of your filter as this will take any medication out of the tank.
Keep an eye on your water quality and after the treatment has finished, it would be worth doing another water change.
Your filter needs a chance to mature so I wouldn't add any more fish for quite a while.
Forgot to mention: Don't feed the fish for 2-3 days as leftover food can also cause problems with your water quality.