Most of these situations can be resolved with a lot of water changes. If it has anything at all to do with the overstocking, water changes will help. If it's an outbreak of columnaris, you'll need meds. But since you already know you're overstocked and have still avoided telling us how much ammonia is in your tank, start doing a 20% water change every day until you get some more solid clues as to what's wrong.
Are any of the guppies showing signs of stress like clamped fins, fuzzy spots, red gills, acting "clumsy" and such? Are they gathered at the surface to breathe? Do you use dechlorinator?
This. What kind of filter is on their tank and what is their current temperature?
Are any of the guppies showing signs of stress like clamped fins, fuzzy spots, red gills, acting "clumsy" and such? Are they gathered at the surface to breathe? Do you use dechlorinator?
