Hi Can I have you expert opinion on what this might be. It looks someone has drawn a little cluster of circles on the glass with pencil. The photo doesn't show it particularly clearly. Scrapes off easily. First thought was diatoms as this tank is new but ammonia has consistently read 0 since setup. The little lines are air bubbles from the filter. I was checking flow round the tank and forgot to turn them off for the photo Tank 90l lighting. 24w t8. 6hrs/day co2 1ml easycarbo per day ferts. 1ml tpn+ per day 2 mature internal filters taken from another tank 1200l per hr combined tank has been setup a week. Ammonia and nitrite constantly 0 nitrate highest 30ppm 2 x 30% water changes so far as I have young rams in the tank.