can round worms survive without host, for how long?

[fish nanny]

Hi all, does anyone know how long round worms can live without a host. I have a tank set up for fry, but had to use it as a hospital tank for a fish. He had been treated for parisites when I got him, but seems as though he had round worms still. He would not eat the worm meds and passed soon after I got him.
This tank will need to be ready for fry soon, it has a PF sand substrate, and planted, I can pull plants up if needed, it is a 20L............What do I need to do to make sure it is safe, can't seem to find anything about round worm life cycle ? Thanks