Hey Sabina i love the picture in your avatar! Is it your dog and a horse? I recently found out about puppy mills and saw horrible pictures , many trash cans with full of dogs. Don't even want to show you that link i saw , so sad. I live in a state where we have a lot of farms with abused horses. When i see pictures, like in your avatar, it makes me happy thinking that at least some people love animals.
I hope your fish doing good. One tip , always make sure you swish/rinse the filter media in the tank water while you doing a water change . Filter has a lot of gunk on them that needs to be washed off. Never afraid to do an extra water change. Water testing involves looking for things like ammonia. However it won't tell you about other contaminants or bacteria levels . So more water changes and filter media cleaning in my opinion is very important.