Vinegar eels, Tubatrix aceti, are one of those freshwater food cultures that I think too few people with aquariums have learned to use and appreciate. That said, they fill an important niche in fish culture, being larger than infusoria but smaller than microworms and baby brine shrimp.

One purveyor of live food cultures, the Bug Farm in San Rafael, California suggests the following uses for vinegar eels:

• Betta fry when newly hatched
• Corydoras fry when newly hatched
• Apistogramma fry at free swimming
• Killifish fry when newly hatched
• Gourami newly hatched to 1/4 inch
• when fry are too small for microworms
• when fry are too small for baby brine shrimp

I prefer them over microworms for one simple benefit: unlike microworms, vinegar eels can live in freshwater for an extended period of time (possibly up to a week) and swim in the water column where baby fish are feeding (in comparison, microworms sink and die quickly). Vinegar eels are harmless nematodes, non-parasitic, and their lively wriggling actions make them highly attractive to aquarium fish fry.  (See Video link at bottom of this page.)