Hi all just cleaned out my plastic 20l tank its about 5 years old and iv always been as carefull as posible when filling . the amount of black stufff i cleaned out was full on, i was un able to get it all out. i also checked the filter on the inside of the tank if any one does this, on my tank it did not come all the way out i looked at it threw the tank filler. glad iv done it now think ill do it once a year cheers all.