I don't culture tubifex intentionally. But I do notice that my tubifex worms survive for weeks (though I don't see them reproducing in large amounts) in aged KTP water where I grow my fry out. They just survive on organic matter / uneaten food in the tank. Same goes for daphnia in the same conditions. I normally introduced a squirt or too of daphnia / cyclops every two weeks and they seem to maintain their numbers for quite long. The fry, on the other hand, are constantly happy and full.