You can use a nitrate remover in the external filter its something you can buy to put in it also amquel chemical is very good at removing nitrate the new filter you bought have pipes that go outside the tank or does it sit inside the tank don't turn off the underground filter until you habuve...
I wouldnt change the substrate I'm suprised you didn't cause an ammonia spike when you did clean it I would get an external filter run that with the ugf and use a nitrate remover and phosphate remover in it that should remove the dirty look to the water and the high nitrate levels
Hi I'm new here but been in the hobby a while. I have just gotten a triggerfish and I put him in my sons tank it's is only a few months old over night something happen cloudy water and on his side not moving and gasping I thought to be honest that was the end of him anyway I took him out and put...