I've noticed that everyone seems to be running open top barrels for their biofilter, meaning, the media is exposed to light.
Correct me if I am wrong, but should not the media be in total darkness 100% of the time?
I keep mine dark. I cut the top of a blue barrel off with a reciprocating saw where it narrows and then flip it over. Makes a perfect snug lid. However I do allow a slight crack away from the nearest light to allow carbon dioxide to escape. I also keep light dim in my fish room 24/7 with a dimmer switch.
I believe an open top is functional but my darker mbbr's increase the carrying capacity of the media especially the nitrobacters. Bacteria does not like light and an extreme example is ultraviolet light which kills bacteria.
Btw I no longer feed my mbbr's the filtered water from the top of clarifier barrel to the bottom side of the mbbr barrel. I was having issues with mulm building up in the bottom sweep of waterline from the top of the clarifier barrel to the bottom of the mbbr, which reduced flow and subsequent return flow to the fish tank. And increased pressure in the bottom of the barrel also reduces flow. And it's a PITA to disconnect the water line with water in the tank to clean out the water line! Water flies out in the second or two it takes to cap it!
I now drill a hole in the middle of one of the bunghole threaded caps of the lid of the mbbr and push the end of the water line snugly into the hole just a couple of inches. Then attach a hose clamp on the inside on the hose to assure it doesn't pull out. The end of the waterline does not make contact with the water or media in the mbbr barrel. If it does it can mess up the pattern of medIa flow.
Since the bunghole caps are positioned on the outside edge of the lids, the flow immediately goes down with the media in the barrel which goes down on the outside edge and up in the middle.
