I hope you've called your vet about this by now!!

I just found some worms like that in my rainbarrel -- lots of them. They are 1-2 cm long and the color of spider mites or deeper red than that. This rain barrel is a bit nasty as it's full of yellow goop coming off the gutters which are full of maple seeds. (I just bought this house and the guy doing the roof job for me is not available yet.) I never saw this in my rainbarrels at the other place for 3 years.

I'm in England but we get red worm like larvae about 15mm long which are 'bloodworms' or midgy larvae, the brown ones are mosquitos. They hatch in still water after around 4 days and feed off debris.