One of my 6 goldfish has started lying on the bottom of the tank. This has been going on for about 4 days now. She does swim around at times with the others but always ends up on the bottom again. I tested the water and ammonia, nitrate and nitrite levels are all ok. I did add some aquasafe 2 days ago too just in case. I have been feeding peas for the past 2 days but no change either. Fish tank is 550l All fish are 2 years+ and I have had no new additions. Should I do a partial water exchange although the levels are ok? Or what else could be wrong with this particular fish as the others seem fine?