I'm posting in here as an article I've just read says they are found in tanks with poor filtration, amongst other reasons. ( I don't know how to post a link, it's from 'fish as pets aquatic news')
I'm currently cycling my 55l tank and have found a load of tiny thread like worms on the glass. Googling has lead me to the conclusion they are detritus worms (harmless) rather than planaria (not harmless).
But the population explosion is a sign of bad conditions in the tank apparently.
There are no fish, so no over feeding is going on.
There is quite a lot of the bottom layer of substrate hanging about on top of the sand since I planted some plants a few days back. I tried to remove it yesterday by vacuuming, and that stirred it up into the water column, made it very murky for a while.
Do you think that is the cause?
The other possibility is poor oxygenation, do I need to get an air stone?
The minute I turn the pump rate up, the tank overflows, so I'm keeping it turned down atm.
Any advice would be most welcome.