I have 6 discus, got them all at same time almost three months ago. We're all same size at te time, now two are probably 10cm body, 2 at 8cm, 2at6cm. Tonight diamond came out and all looked good, until the big pigeon did two rubs against a plant just as the light went out. They seem to only do it at night time between the first light going out and the second one (they are on different timers. 300l tank and mates are cories, rummies, bristle and harlequins. All other fish seem fine, although cories still do their weird unconscious thing at times as per my other older post