Dwarf puff, Big Mama, died today. A few days ago we noticed a spot but wasn't sure if it was an injury or something else. She stopped eating a day ago and just hung out on the bottom of the tank Today, she didn't move at all and I noticed another faint spot on her back. We moved her to a separate container since we have another dwarf puff, Lil Mama, in tank. The other puff has no spots and is acting fine. After we took Big Mama out we did a 75% water change and started treating with Erythromycin. I have attached a pic of the spot by her mouth.
Water: 0 ammonia and nitrite. Nitrates 25 ppm.
Tank size: 20 gallon long, 2 Hob filters (1x ea

[/img] aqueon 30 and aqueon 70)
Feeding, wc sched, meds: Feed 1 to 2 times a day. Live black worms, white worms, and snails.
Changes in last week: Nothing has changed except Big Mama not eating and lethargic.
How long has tank been up: Tank has been up for 2.5 years.
How did you cycle tank: I cycled the tank using media from other 10 gallon tank and dosing with Fritz ammonia until the water parameters were steady for 2 weeks.
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