Im wondering about diatom filters and how they are good for the aquarium?

I have this 55 gallon aquarium with two oscars a jaguar cat and a placo i just trnsfered them from a 30 gallon and the tank was fine when i first put them in but when i checked it in the morning in was murky all the chemicals and everything are fine i am the temp is a little on the cooler side about 74 and i am using a hang on the back filter i cant seem to figure out what is wrong with it i was doing some research and i heard about diatomatious filters and i was wondering is it the whole filter system or do they just fit into a canister filter and if they really work?