There's a lot more factors that play a large role in water clarity then just filtration. I'm guessing in this particular case its simply a very well established aquarium with a long standing unchanged bio-load. A Uv sterilizer can be eliminated due to the amount of hair algae on the wood. My guess would be the use of Carbon, Heavy Filtration and a solidified bio-load. If he was doing large weekly water changes that amount of hair algae wouldn't be present. Its nice to see these types of monsters in a decent sized aquarium. Rather then crammed in a tiny 200g. Thanks for sharing Op.