Thusly (chris nuttall) Spake Unto All:

what is 'old tank syndrome'?

Never heard of it in freshwater, but in sal****er it is an observation
that after several years of flawless operation aquaria tend to slowly
decline. "Restarting" the aquarium with fresh live rock and sand
brings it back on track. Ron Shimek has propsed that OTS is caused by
heavy metal build-up in the rockwork - at the pH's seen in marine
aquaria, the heavy metals found as contaminants in the salt mix
precipitate on limestone.