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you cant really see anything in the video but if you see a white dot on the tail its something called Ick. not good but treatable at early stages. its a parasite on the fish that chews at it and the white dot that you see (there will be more LOTS more if u dont treat it) is a pus bubble that forms over it. at about 80 degrees the adults will fall to the bottom of the tank and "give birth" to little babies that float around the tank but these babies die easily so keep the temp up!!
all you have to do is slowly raise the temp to 82 degrees and keep it there for 2 weeks or so. the temperature will kill off the parasites.
chemicals are unneeded in this case but things like ick cure and ick II etc kill the parasites in a harsher way plus sometimes kill other bad creatures... when using chemicals u always come across the problem of potentially overdosing...
the choice is yours but im sure most will say just to raise the temp. good luck!