Originally Posted by
kntry
That little one looks very thin. That is an ulcer and a pretty serious one. You need to clean the ulcer with peroxide on a cotton ball being careful to not get it in the eyes or gills. Rinse, pat dry with a paper towel and put a mixture of antibiotic and Tri-Cide Neo (order online) and then sprinkle denture powder over it to seal the meds in.
When is the last time you took the Matala out of the barrel and cleaned it? Is the filter cycled? Lava rock is not the best choice to use underwater as it clogs very fast and gives parasites and bacteria a place to grow. Do you have a bottom drain? Is the bottom clean or is there muck on the bottom? You have a large fish load in 400 gallons. You would do better to do 2 water changes of 25% spaced out over the week.
You said there are no new additions but all these fish are very small. Were they all bought from the same tank, at the same time?