Froozen food 99% of the time still contain life eggs of other guests, like planaria and or other kind of eggs. If you only have fed twice with bloodwurm it probably aint from this, yet. But you probably will get some in the future if you keep feeding froozen food. Also plants can bring all kinds of things, wurms, hydra, cyclops etc. I've never seen a patern like that in a tank, but it surely looks like the tracks of some kind of wurm like creature. If it never surfaces can only mean one thing 😉 it doesn't like light, so keep your eyes open when lights are off and or look under rocks. You do not need to lift your hardware, but temporarely could put a flat stone in the tank en look under there after a while. You might find what it is..
It's relatively common to have wurms and or some other kind of creature in the tank next to fish and shrimps as long as they are not parasitic to the fish etc. there is no need to alarm and probably serve as extra fishfood. Me too i found saw a 4 cm long white wurm crawling on the glass in a 6 week old tank. 🙂 must have come with the plants or in the bag of soil.
Many years ago i sometimes did feed my cichlids with some kind of life wurm it wasn't tubifex i forgot it's name, could be bloodwurm i don't remember. But i found some years later back during a clean out still living in the soil, rather long wurms. 🙂
