Red Lesions on my Fish! Help 

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What is the water volume of the tank?
10 Gallons
How long has the tank been running?
2 Months
Does it have a filter?
Does it have a heater?
What is the water temperature?
71-74 Degrees Fahrenheit
What is the entire stocking of this tank? (Please list all fish and inverts.)
7 Small Zebra Danio (3/4 to 1 1/4 inches) and a Small Siamese Algae Eater(3 Inches)

How often do you change the water?
Every Two Weeks
How much of the water do you change?
What do you use to treat your water?
Seachem Prime
Do you vacuum the substrate or just the water?
I vacuum the substrate and the water.

*Parameters - Very Important
Did you cycle your tank before adding fish?
What do you use to test the water?
API Freshwater Master Test Kit
What are your parameters? We need to know the exact numbers, not just “fine” or “safe”.

Ammonia: 0-.25
Nitrite: 0
Nitrate: 5
pH: 8.0

How often do you feed your fish?
Once Daily
How much do you feed your fish?
As much as is eaten within 1-2 Minutes
What brand of food do you feed your fish?
TetraMin Tropical Flakes and occasionally a Hikari Algae Wafer
Do you feed frozen or freeze-dried foods?

Illness & Symptoms
How long have you had this fish?
3 Months
How long ago did you first notice these symptoms?
1 Month Ago
In a few words, can you explain the symptoms?
Isolation, Lethargy, Loss of Appetite, and Ulcers/Sores
Have you started any treatment for the illness?
Was your fish physically ill or injured upon purchase?
How has its behavior and appearance changed, if at all?
See below

Explain your emergency situation in detail. (Please give a clear explanation of what is going on, include details from the beginning of the illness leading up to now)
In December, we noticed that two of our Danio were swimming in place and avoiding interaction. Within a week or two, they also stopped coming to the surface when we fed them. Yesterday, I saw red lesions near the analfin on one of them, and today the other one was briefly laying on the gravel.
We haven't seen any signs of weight loss or worms.


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3 years
How did you cycle your tank? Either the ammonia reading is false, or your tank isnt cycled. In a cycled tank, there shouldn't be any ammonia present unless it's in your tap and you just did a water change. I would most definitely say its ammonia poisoning. Daily 50% water changes and dosing the entire tank volume with seachem prime during the water change until the tank is cycled. And definitely dont replace any lost fish yet.
Edit: btw in a tank that size or any size imo you need to do water changes weekly. I always recommend 50% and in a 10 gallon that probably only has about 7 gallons in it, its not a pain to do 3.5 gallons a week. The smaller the tank the more frequent water changes are required
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I used one side of my dual sponge filter to instant cycle the tank, but still tested it by adding ammonia and it processed out fine.
The ammonia reading is at least sort of false because we have never seen what looked to us like zero, but zero was the only logical explanation after it was processing 1 ppm until it got to .25, where it seemed to plateau. That's the reason I listed 0-.25.
None of the symptoms seem to suggest ammonia poisoning.
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