Is this cottonmouth?
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Colin_T
Aquarium Advice Addict
Hi and welcome to the forum 
This is not cotton mouth disease (mouth fungus, aka Columnaris).
The fish has a new disease, which appeared a few years ago and we think it's a virus that causes the bottom lip to balloon out. It seems to be restricted to neon tetras, cardinal tetras, and closely related species like the green neon tetra. It does not appear to spread to guppies or other unrelated fish so it shouldn't cause problems to the guppies.
The lump does get bigger and eventually the fish has trouble eating. When that happens we suggest euthanising the fish so it doesn't starve.
On a side note, neons do best in larger groups consisting of 10 or more of their own kind. However, that also depends on the tank size. Generally the minimum size tank for neon tetras is around 30 inches but they can be squeezed into smaller tanks if needed. The smallest tank would be about 18 inches long for a group of 6-8 neons. You need at least 6 neons (preferably 10 or more) for them to act naturally and be less stressed out.
These fish naturally occur in groups of thousands and when one or two become isolated, they stress out really badly even if there are no predators around. Their natural instincts say they need to be with lots of others and this should occur in an aquarium.
Perhaps the next time you are near the pet shop, swing by and grab 6 more neons. Check them at the shop before you buy any. You don't want any fish if they have cream, white or grey patches over any part of their body, head or fins (external protozoa).
Avoid the fish if they have white lips (Columnaris).
Avoid the fish if the blue or red line is faded and doesn't look normal (neon disease). These are all signs of neon disease, mouth fungus and external protozoan infections and will kill these fish quickly. If one fish in their tank shows any of these symptoms, do not buy anything from that tank. Go back a few weeks later and see how they are doing.
If you post a picture of the entire tank, we might be able to offer suggestions to help improve its appearance and reduce the stress on the fish.
This is not cotton mouth disease (mouth fungus, aka Columnaris).
The fish has a new disease, which appeared a few years ago and we think it's a virus that causes the bottom lip to balloon out. It seems to be restricted to neon tetras, cardinal tetras, and closely related species like the green neon tetra. It does not appear to spread to guppies or other unrelated fish so it shouldn't cause problems to the guppies.
The lump does get bigger and eventually the fish has trouble eating. When that happens we suggest euthanising the fish so it doesn't starve.
On a side note, neons do best in larger groups consisting of 10 or more of their own kind. However, that also depends on the tank size. Generally the minimum size tank for neon tetras is around 30 inches but they can be squeezed into smaller tanks if needed. The smallest tank would be about 18 inches long for a group of 6-8 neons. You need at least 6 neons (preferably 10 or more) for them to act naturally and be less stressed out.
These fish naturally occur in groups of thousands and when one or two become isolated, they stress out really badly even if there are no predators around. Their natural instincts say they need to be with lots of others and this should occur in an aquarium.
Perhaps the next time you are near the pet shop, swing by and grab 6 more neons. Check them at the shop before you buy any. You don't want any fish if they have cream, white or grey patches over any part of their body, head or fins (external protozoa).
Avoid the fish if they have white lips (Columnaris).
Avoid the fish if the blue or red line is faded and doesn't look normal (neon disease). These are all signs of neon disease, mouth fungus and external protozoan infections and will kill these fish quickly. If one fish in their tank shows any of these symptoms, do not buy anything from that tank. Go back a few weeks later and see how they are doing.
If you post a picture of the entire tank, we might be able to offer suggestions to help improve its appearance and reduce the stress on the fish.
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