It sounds like your aquarium is acidic with low pH levels
How to be sure
Your test results show acid or low pH levels (below 6.5) and your aquarium generally looks ok – this is an invisible water quality problem. Your fish won’t like it – they may appear pale or dark and shimmy, scratch, jump and flick against objects in the aquarium. Rapid gill movement and breathing at the water surface are also signs of your fish’s discomfort.