Koi upside down with red bellies

[jen626 3]

Hi, I am posting this for my step-son who is trying to diagnose and save some of his koi.  He has a huge Koi pond outside that's like 5500 gal, but he has a 500 gallon tank inside where he keep smaller fish until he moves them out into the pond. He has 6 fish that are about 6 in Long in this tank right now, and all of them are acting sick. I know that he added a new fish recently so I'm assuming that it's parasites because he didn't quarantine but I'm wondering if these symptoms point to something specific without him doing a scraping. They have been treated with an antibacterial tank treatment as well as PP.  They started out kind of drift swimming off to the side and their bellies had red spots on them, now they are swimming upside down.

Test Results for the Following:

 Ammonia Level(Tank) 0

 Nitrite Level(Tank) 0

 Nitrate level(Tank) 10 

 Ammonia Level(Tap) 0

 Nitrite Level(Tap) 0

 Nitrate level(Tap) 0

 Ph Level, Tank (If possible, KH, GH and chloramines) 7.2

 Ph Level, Tap (If possible, KH, GH and chloramines) 7.2

Other Required Info:

 Brand of test-kit used and whether strips or drops? API master drops

 Water temperature? 72

 Tank size (how many gals.) and how long has it been running? 500, 9 months

 What is the name and "size of the filter"(s)? Custom filter DIY, it probably filters 3000 GPH it's pretty intense

 How often do you change the water and how much? Based on nitrates but usually 25% weekly

 How many days ago was the last water change and how much did you change? 4 days, 30%

 How many fish in the tank and their size? 6 six inch koi

 What kind of water additives or conditioners? Prime 

 What do you feed your fish and how often? I forgot to ask him this, he's not feeding them at all right now but normally he feeds them once a day and they're high quality koi pellets

 Any new fish added to the tank? Yes, probably how this started was a new fish

 Any medications added to the tank? After they showed the red spots on their bellies he did a PP treatment on all of them and I believe he has an antibacterial added to the tank, I am waiting to hear which one it is

 List entire medication/treatment history for fish and tank.Please include salt, Prazi, PP, etc and the approximate time and duration of treatment.

 Any unusual findings on the fish such as "grains of salt," bloody streaks, frayed fins or fungus? They have red spots on their bellies and are swimming upside down. Before they started swimming upside down they were kind of drifting off to the side and then adjusting themselves

 Any unusual behavior like staying at the bottom, not eating, etc.?

Just the upside down swimming and drifting off to the side before that started

[jen626 3]

Adding that AP General Cure and Erythromyacin have been added to the tank.  PP treatment was done at least twice on each fish.

[Arctic Mama 3,645]

Hi!  What was the duration of the treatment for the General Cure? Erythromycin?

Unfortunately the red bellies are a very bad sign, especially combined with the poor swimming.  It is likely viral or parasitic, given how it spread to every fish in the tank and the water parameters being nice and stable.  This is likely just a really good example of why quarantine is so critical

[jen626 3]

He just started the GC and erythromyacin on Tuesday evening so it has been about 36 hours.  

[jen626 3]

Current picture, some doing better some not

[Arctic Mama 3,645]

Okay, discontinue the erythromycin, but continue the general cure.  I like to dose the full volume every 24 hours, since I do daily water changes in the hospital tank.  Continue that for 14 total days.  

How are their gills looking? Pale, cherry colored, or more maroon? Any surface gasping or excess slime coat?  Do you have aquarium salt or methylene blue on hand?