The tank is a 60l one, I'm not sure of the filter, its one of the ones that hang of the back with the inlet in the tank, we are pretty good with our filter maintainance though, cleaning one half one month and then leaving it for a month or so before cleaning the other half so we don't wipe out all the bacteria in one go.
I know we aren't perfect with our regime but it has worked up till now and we haven't changed anything we do to suddenly cause a change and prior to the last week the pH has always been 7-7.5.