I always feel bad having to break the news to unsuspected fishkeepers
That their aquarium is too small for the fish they have, as in this case,
There is simply not enough water to house the fish you have so that they
Can remain healthy.
The red spots and streaks on the tail are signs of 1) bacterial infections
And 2) parasite infections.
The high ammonia your aquarium you're experiencing is due to an uncycled
Aquarium: please see Aquarium Cycle
sticky note that explains the process.
The stress caused by ammonia in the aquarium can cause disease and parasitic breakouts.
As for treating the ammonia, use a product called Cycle by Nutrafin and
Do not remove the fish or over clean the tank while using this product.
It will quickly cycle the aquarium for you. Over cleaning removes beneficial
Bacteria that is important to keep the cycle going and the water free of toxins.
Treating the infections. I recommend first using Jungle Brand Fungus or
Parasite fizz tabs.
It would be in the best interest for your fish if you moved up to at least a
10 gallon aquarium and a 20 gallon would even be better.
All the treatments mentioned above can be used all at once on all fish.
The fish showing no signs of the illness have still been exposed to it and
Will need the early treatment.
If you have any other questions, concerns, or anything, please let me know.
Good luck and happy fishkeeping