Yellow fish tank

Recently,my a few of my fishes are losing their tail and fins.I thought it was because the barb that i have.That is suppose to be a fierce one.So been feeding more thinking that it might be hungry.Then today went out to family friend's house and noticed that hers was also having the same problem.But she didn't have any barb or fierce fish.Finally.....decided to go to the fish shop at 9.30pm and the person sold me this yellow powder.That's how my tank turn yellow.Looks as though i have not been taking care of it.Sigh...