[Pavel Chaloupka]

well most of the stuff small enough to be ingested that lives in water is good food for fish. Unless its some parasites which is very unlikely given you do not keep any fish in the buckets or a predator like dragonfly larvae. Are the worms swimming? Or they reside on the bottom? It could be something from Lumbriculidae family for example, which would make great food if it comes from non poluted waters as your balcony waters should be 🙂 Most of the harm done by feeding with Tubifex, Chironomus larvae and such stuff is caused by cumulative poisons that come from the localities of colection of the foods. So more likely than not, it could very well be good food . Of course picture would help if you manage to catch some in some transparent plastic jar.