Possible cottonmouth on neon

[CCK]

Its long but thanks for the advice.

I took over a 10 gallon tank for another family in September and have learned quite a bit since then on improvements to make. 

The tank came with 2 neons and 3 ember and a boseman rainbow. As well as some shrimp and mystery snails. 

One neon and 1 ember died within a week of each of each other. I thought this was due to temp drops as the previous person told me that she kept the water around 70 and being the heater was broken plugged it in occasionally for an hour to get the temp back up to 70. I bought a new fuctioning heater. 

The neon that was left was stressing because he lost his friend. My husband went to the store and picked up 3 more neons (oct 22). One of the new neons completely disappeared- maybe by the rainbow- maybe died and the shrimp ate him (who knows). One of the new neons kept picking on the original neon and then the original ended up with what I assume was cottonmouth or maybe just injured from the new neon and the original neon also died. 

After that death I treated the tank with API Furan 2  Once that was complete we rehomed the rainbow to a home were he could have a school and bigger tank. 

I then started working on a bigger tank- now have a 20 gallon long that is cycled and planted. Has an aquatech 20-40 filter and airstone.

We moved the 2 embers and 2 neons over at least 7 days ago and put 3 tablespoons of aquarium salt in the tank to help with stress and their fins weren't looking the best. We moved over the shrimp 2-3 days ago. I did a 50 % water change last night so that we can move the mystery snails over. My daughter noticed that one of the neons now has a white spot on her lip..........

The goal was to get them all in the new tank and then go get more embers and neons/cardinals and put them in the 10 gallon as a quarantine tank so that they could actually be properly schooled in the end. 

What should my step be now?? Take the one single fish and put it in the 10 gallon and treat it? (what should I treat with?) Even though that might stress her being all alone? I can't do both neons as the other one gets stressed when its just him and the other neon together - he doesn't calm until the 2 embers are with them. 

Other information. Ph 7.4 (steady), temp 75, have been testing levels with the API master kit

Edited December 29, 2020 by CCK
added more information

[Colu]

I would qt the fish and treat 

[Hobbit]

You could also treat everyone where they are in the 20 long. You’d have to use more meds but if you don’t mind doing that you wouldn’t have to worry about anyone being alone.

Can you take a picture of the white spot and post it?

[CCK]

So she looks better now. In regards to the spot on the front. Both of the neons do have some fin issues so I want to treat them still anyways.

I am waiting for the trio medications from aquariuam co-op and and going to be ordering some new fish to get mine schooled appropriately. Once the new fish get here can I just medicate the entire 20 gallon with the new fish in it -even if there are mystery snails and shrimp? Or should I just take all the fish and put it in my cycled 10 gallon. 

[FrozenFins]

1 hour ago, CCK said:

So she looks better now. In regards to the spot on the front. Both of the neons do have some fin issues so I want to treat them still anyways.

I am waiting for the trio medications from aquariuam co-op and and going to be ordering some new fish to get mine schooled appropriately. Once the new fish get here can I just medicate the entire 20 gallon with the new fish in it -even if there are mystery snails and shrimp? Or should I just take all the fish and put it in my cycled 10 gallon. 

I wouldn't see the harm of medicating the fish if there is only shrimp and snails. If there are other fish I would use your 10 gallon.