SUPER TOXIC water for my goldfishes!

[FlyingChicken]

All my 19 goldfish in my newly setup 4x2x2ft tank has been badly infected with the deadly internal bacteria infection. Ulcers and open sores can be seen on their mouth and belly. Water turns brownish, foaming and damn cloudy.

2 wks ago, my tank is still spanking clean but now things have changed and I don't know what happen. All I did recently is to put some hornwort at my sump filter and I don't think that simple action could have cause such a nightmare.

Anyway, I've transfer all my fishes back to my small and cramp 2ft tank for medication and they are recovering well.
Now, I'am thinking whether to keep the bacteria in my sump filter or restart everything again from scratch...

[vinz]

Putting hornwort should not do this. Did you add anything else, or do anything else. Like, did you add fertiliser? Did you do a water change just before this happen? Just list out anything you did to the tank just before it turned bad.

Give the tank a few days, it might clear if it's the same situation as mine. Read my thread.