Can anyone give me some reassurance or advice; We have a 70 litre fish tank with one fancy and one common goldfish. For a while now the fancy goldfish has been spending a lot of time on his back. I have changed their diet from flakes to pellets and he has made an incredible recovery! However, since they have gone to pellets, the tank smells - not fishy, just yucky. I've got the test kit that checks the PH level, ammonia, nitrate and nitrites and all are absolutely fine. I carry out a 25% water change once a fortnight and test regularly, but that smell is still there! I'm assuming it has something to do with the pellet food, but is there anything I need to be doing to combat it or should I just live with it?

Thanks for any help!