Need help, koi died

Hello, this is my first post here. My mom has a cold water tank that used to have 5 fancy goldfish, and 3 koi fish. She added in a black koi less than a week or so. The black koi turned gray and had red gills so my mom returned him, 2 days later both kois died. After she changed her water, one koi immediately started to swim upside down and side ways and lay on the bottom of the tank, he died within minutes. The other koi did the same, but lasted overnight and died in the morning. We are not completely sure why it happened. The goldfish are completely fine. Below is more info on the tank.


65 gallon cold water tank.
Temp around the low 70s.
Tank has been running for a couple months now.
No heater, two marine land filters.
5 fancy goldfish, 3 kois(used to be)
Changes water everyday due to high nitrites.
She changes 70-80% of water everyday 1-2 days.
She uses 2-4x dosage of seachem prime.
She barely has any gravel, 95% is bare bottom.
She didn’t understand cycling, so she’s in the middle of a fish-in cycle.
She uses API freshwater test kit.
Ammonia 0ppm, Nitrate 10-20ppm, nitrite ranges from 0.50-2ppm, PH 7.6
She feeds her fish everyday
She feeds frozen veggies and frozen meat like blood worms, shrimp, etc.
She feeds twice a day, first time pellets, second time frozen food
She had two kois for a couple months, black koi fish for a couple days.
Swimimg erratically, no control, sideways and upside down, and lays on bottom of tank.
No fish was ill, or injured when we first got them, black koi got ill after couple days, 2 kois got sick after water change.

MORE INFO.

My mom has had trouble with the fish-in cycle. She’s been cycling for about 9 weeks but still has a dangerous amount of nitrites in her tank. It ranges from 0.50-2ppm. Past couple days it’s been 0.50. She’s put bottled bacteria, added java ferns, anubias’, swords, photos, monstera, and deliciosa, the last 3 were grown in soil and added them in in hopes to suck in the toxins. She’s added api quick start, stability, and fritzyme 7. She got seeded filter media from a fish shop the same day the 2 kois died after the water change. After the water change, the water was cloudy. Koi died, she removed the seeded filter media and the plants she’d grown in the soil, put back her old media and changed the water. Checked parameters, nothing was out of the ordinary besides nitrites. When she changed the water the second time, the water was much clearer. Can you please help me figure out what could’ve went wrong?

 

That’s to much waste for that small tank. Koi need a pond so they must be returned to the store because they can’t live in a 65 gallon tank and 5 goldfish really shouldn’t be there either. Maybe 3 With that amount of waste you need to be doing daily 50% water changes maybe even twice a day to keep the parameters in check

 

UPDATE*
3 fancy goldfish, 1 comet goldfish, 3 Koi.
The kois died suddenly, immediately after water change, seeded media was only there for a couple hours.

 

how many fish are currently in the tank?

did she qt the new koi? always qt new fish, if goldfish or koi, need to treat for parasites.

were there any physical sypmtoms on the koi? bloating? pineconing (scales sticking out)? gasping at the surface? red bloody fins?

Koi get to 24-36 inches long.

comet goldfish usually get to around 12" long in a tank. 4 fancy and 1 comet is doable, might need to do water changes every 4-5 days depending on the nitrate levels once your tank is fully cycled. do NOT add any more fish, she is at MAX stocking with the 5 goldfish.

speaking of cycle. what size are the filters? and what filter media is she using? take a look at this filter media guide Diy Media Guide For Top Fin Silenstream, Aquaclear And Other Hob Filters | Aquarium Filter Forum | 385506

it should not take a couple months to cycle a tank unless the filter media has been changed out, like with factory filter cartridges. or if the filter media has been rinsed in chlorinated tap water.

 

how many fish are currently in the tank?

did she qt the new koi? always qt new fish, if goldfish or koi, need to treat for parasites.

were there any physical sypmtoms on the koi? bloating? pineconing (scales sticking out)? gasping at the surface? red bloody fins?

Koi get to 24-36 inches long.

comet goldfish usually get to around 12" long in a tank. 4 fancy and 1 comet is doable, might need to do water changes every 4-5 days depending on the nitrate levels once your tank is fully cycled. do NOT add any more fish, she is at MAX stocking with the 5 goldfish.

speaking of cycle. what size are the filters? and what filter media is she using? take a look at this filter media guide Diy Media Guide For Top Fin Silenstream, Aquaclear And Other Hob Filters | Aquarium Filter Forum | 385506

it should not take a couple months to cycle a tank unless the filter media has been changed out, like with factory filter cartridges. or if the filter media has been rinsed in chlorinated tap water.

she has not qt the new koi, just plopped to right in. the koi was losing its color and had red bloody fins. She has two filters that are the marineland 350 with biowheels, she is using filter floss and active carbon, and she only changes the carbon media and she does “change” the filter floss, but she swaps them with the two other tanks she has.

 

hopefully, it was a koi only specific pathogen. ammonia would have similar symptoms, but you said it was 0 ammonia. Nitrite is blue blood disease aka methemoglobinmea. bloody fins could be an internal bacteria/viral infection leading to septicemia. really not enough info for a diagnosis, all we can do is guess without a lab test.

in my previous post i linked the DIY filter media guide. she should look into adding some of the reusable bio media such as Biomax, Matrix, or Biohome.