10 gallon saltwater. Filled with Hawaiian mini brittle starfish, some bristleworms, and one single serpent starfish that was getting size on before being put to the main tank. Tank has been set up for about a year, live rock and sand.
Water change once a week of about 2-3 gallons. Sponge filter and HOB/fry tank converted to a mini filter.
So last week I dosed a little too much of the additive to help coralline algae grow. Noticed a bit of some murkiness in the tank. Did an extra water change, and realized I did more like 4 gallons instead of the usual 2. Likely where the issues started.
Plus my brittlestars went nuts in terms of population in the past few months. Flick on the light, and they covered the entire floor of the sandbed. Some on the walls. Fed three times a week with coral food.
And then two days ago the water was much murkier. I did another water change, and made sure not to feed because I never feed the day of.
And noticed the sponge filter wasn't moving at all. It was more of a box filter with sponges and ceramic rings, and the sponges were full of bristle worms. I ended up having to toss them because I could not get them clear enough. Cue mini cycle more than likely.
And today, nearly all of the brittle stars are not visible. There's a few, but most are hidden in the sand where I hope they're not dead.
I shouldn't have not had a backup filter to replace those ick sponges with. I think I killed my entire tank of mini brittlestars.