New pals in Tank

Today when I woke up and found out one of my fish, Sebastien
is not in good condition. It's showing red blood vein on its tail because of high nitrite, which is toxic to fish. I did a quick water test and the result was horrified!
. The tank has passed through the Ammonia stage (0.25ppm now) but now the Nitrite level has gone sky rocket (2ppm!)! Not just that, there are getting more brown patches everywhere in the tank too. So I went out lunch time and bought a magnetic glass scruber, and 5 shrimps! Hopefully the shrimps will help out eating the excess food that sunk to the bottom of the tank.

Got home, scrubbed the tank, got all the big pebbles out because the food sunk into the bottom and the fish can't get them. So I took them out and did a 50% water change. I was please Sebastien seems a lot happier after I did the water change. I got a small sachet for free from an Ebay buyer who I bought items from. He recommends it which meant to turn the water tank into crystal clear within 10 mins! I was bit skeptical what was inside the sachet and didn't use it. But tonight the water was still murky after the half tank changed, so I have decided to give it a go. I mixed it well with water and poured into the tank, after 10 mins it didn't show much of a effect so I just ignored it. But I came back hours later after watching a movie and I was amazed how crystal clear the water is! I just hope now the fish won't die from the chemical or whatever it is. will see tomorrow how sebastien is.