I have just got 2 gold fish not small not also big and a small pleco ! but

the water always get cloudy? my tank is more than 80 liters with a filter written on it Q.max 600L/H ( as i dont understand how it works , i have just bought it ) and an air pump also some decor in the bottom ! hmm oh yeah also the seller gave me a crystal small things he said it is for the water to reduce the Cl in it ! , and some blue liquid for them if they were ill , it's written on it the same !! so am i going in the right direction or wrong ??
oh am srry but also the small pelco got white dirt on it ! i dont knw why it got so dirty , i dont feed the gold fish many , i only feed them one of their small food ( i dont knw the name it's small red soo tiny food ) for each of them one day and leave the next and so on !!

also the fishes go up to the surface of the water and get air ! i dont knw why ?