i did go back, you had a lot of foam and murky water after a 25 percent exchange and maybe a blackout. the foam indicates a high organic load, possibly spurred on by a crash on oxygen or something wrong with the change water.
either way this means you mustve had a decent biomass of stuff living then dying in the tank.
what i would like to see on home fish tanks is a filter set up with a swirl separation unit with a tapered bottom so that muck can be drained off as opposed to staying in the tank and growing things you dont want. then the water can go direct to your media. i hav these on my 2000 litre recirc tanks and i tell ya i tap of so much stinky crap every few days and it makes me shudder to think of what it used to do to my water before i had the ability to tap it off to waste.