Originally Posted by twentypoundtabby
Sadchevy has it right. Old Tank syndrome is when water is only topped off for a long time. The inhabitants gradually get used to a lower pH and poor conditions, but new stock added do very poorly.
^This. In addition, a fish moved from a tank like this to clean water may be shocked, and possibly die. Suddenly attempting to perform even a moderately sized water change may shock fish in the tank, and raise the pH enough that ammonia buildup becomes toxic, resulting in mass deaths. People who experience this often come to the conclusion that water changes are bad for fish, not realizing their lack of maintenance was what got them into this predicament.
I do 50% weekly water changes. So no old tank syndrome is possible, or worries about excessive nitrates. Still, in some cases the mulm will eventually pile up until there is an unbroken layer - either on top of the substrate, or just below (as seen from the front of the tank). Which blocks circulation into the substrate. If I let that go too long I do, eventually see a decline in plant growth. Had a nice carpet suddenly die off once, I'm pretty sure the substrate went anaerobic and started producing hydrogen sulfide gas. Which is far more toxic than nitrate!
So a few times a year, if things start looking too mulmy, I'll do a gravel vac. Not to get everything squeaky clean, just remove some buildup, and stir things up. For things that can't be vac'ed (like carpets), I'll go at them with a pipette to blow out some mulm. Or inject a dilute H2O2 solution into the substrate to break down mulm.