I have 7 discus of about 7cm for a month now. Yesterday I noticed that the one is dark and spitting his food out. I tested the water for ammonia and nitrites but nothing. I then did a proper cleaning of the glass, plants and substrate plus replaced the usual 50% water. I do a 50% change and subsrate cleaning twice a week. Today when I arrived home 4 was dark in colour only the bigger ones weren't. When feeding them one didn't eat and two was spitting out the food. The tank has been running for 4 months now without cleaning the filter. Could this be the reason?