The others have excellent suggestions of possible issues to look into but I want to add something that some people may overlook but should be more obvious.
When you do a water change, do you add Prime or some type of tap water conditioner (if you're using city water) to the fresh water? I ask this because when I was new in the hobby I used to remove water, add fresh water, and THEN add the tap water conditioner into the tank! I learned that was a huge NO NO. If you're conditioning your water make sure you put the conditioner into the bucket of fresh water, not into the tank. I nearly killed an entire tank of fish once because I did a big pwc and I added the clean water without conditioner (I used to put the conditioner into the tank after I added the water). One fish rolled over and floated to the top dead within 30 seconds of me adding the water. Some were gasping at the bottom and laying on their sides and others were at the top gasping. That was a night I'll never forget!!!