your in luck, I have an ac300 too! So it's probs an abundance of fish gunk and snails etc... So what you should do it take the bio filter an float it in your tank or water from your tank so not to kill the bacteria. Basically go to a sink or tub or outside an take apart your filter. There is an area where the motor is that, once you empty the water out, should be able to be unscrewed slightly and pull it out( in photo). Then u can use a q-tip to clean the impeller and get all that stuff off it or even rinse out the motor really good. You'll be surprised by the gunk in it! Then you just screw it back in and put everything back together and put he water in and begin filtering! It also wouldn't hurt to clean your intake tube with a new tooth brush or a pipe cleaner. If you need any extra help just ask, goodluck and hope this helped!
