Hey so as the title says...the tank my fish had before had no issue at all. Stayed lovey, the tank i upgraded too, 10 months on the gravel has algae the decorations are covered in algae it looks natural but not pretty imo. The only thing different is in the old tank i had an air stone? I do two water changes a week, my ammonia is 0 my nitrite is 0 and my nitrate about 10/20 Could the airstone of really made a difference?! Im upgrading again this friday (yay for growing goldfish haha) and want my new tank to stay nice! Ive brought filter media which is meant to.help. aswell as a powerful external to replace my internal filters... first pics is my old tank how it looked and my current one now. Any help please?! Thank you!