New fish sick!

I so know better to have a quarantine tank yet I don't. I just have no room.
I had a sick honey gourami that I have since removed as it just wasn't doing good. No idea why shows no signs (LFS has since removed all there's they were selling).

I purchased 3 rainbowfish (they only had 1 female so have to wait for more in stock).
One of them looks like it has white on the bottom of the mouth. And now it looks like my remaining honey hourami has 2 little white spots on its tail

I don't know what or how to treat ...

In the tank;
3 rainbow fish
1 honey gourami
5 false juliI corys
1 (very small) bristlenose pleco

Cycled: yes
Tank age: 3mos (transfered everything from old tank (1yr) to this one)

PH 7 - 7.2
Ammonia 0
Nitrite 0
Nitrate 10-15


* Not the best photo's cause there just so fast!
Hopefully it's enough though

 

What size tank is this?

 

30g

I added the rainbow fish 4 days ago

 

I don't know what could be wrong with the rainbows, but your gourami might have ich- a picture of him would be helpful. To treat ich, you want to slowly raise the temperature of the water to the low 80s, and do daily 50% water changes. The ich shouldn't take too long to treat, because it looks like you've caught it early. Remember that even if ich looks like it's gone, you want to continue to do the daily water changes for at least another week, or it will likely come back.

 

Your Rainbow Fish has Cotton Mouth aka Columnaris. Try treating with Melafix and daily water changes.