guppy question

[pandafishowner]

ok, another question: i have what look like little particles in my tank now. i did a water change last night, about 30%, added back new water, and i still have these things. i've seen the guppies go after the things too. some look like little things that dart across the tank, while others look like little particles of dust?? know what they are? could it be because the filter needs to be changed again? i'm going to crack this filter cartridge open and leave a piece of it in my tank when i replace it.