Guppies Dying Please Help!

[Mandi7891]

I have a 30ltr tank which did have 12 guppies, this is still a newish tank was set up 3 weeks ago now. Fish have been healthy but a week ago I noticed few had pink gills upon checking levels ammonia was high, spoke to a fish shop who gave me a ammonia remover, I needed two lots of treatments and the levels went down.
Again for last week fish been absolutely fine, feeding well and was expecting most of females to give birth anytime, then last night I noticed some had white flakes on the rear tail, this morning within the last hour 3 have died, there was 12 in tank but now only have 9, at least one of these 9 is looking poorly at the moment it's floating vertically, the others are very quiet and not really moving much.
Levels are
Ph 6.0
Ammonia 0ppm
Nitrite 0ppm
Nitrate between 20ppm and 40ppm
Temp 26.1
Someone please help me save the rest of my fish,
No fish have been added in 2.5 weeks
Have tried adding pic but isn't letting me.

 

[star4]

I take you set up the tank and added the fish. You are now doing a fish in cycle. Info on what to do here.
http/www.fishforums.net/index.php?/topic/224306-fish-in-cycling/

 

[Mandi7891]

The water was from my brother in laws tank, so was the pump, so all been cycled, we left it a few days before introducing any fish, took picture to local fish shop and they have said fin rot so added a treatment to it, hoping this works.

 

[star4]

Sorry from your post it read the whole tank was newly set up, my mistake. Fin rot can be brought on by stress, I would suggest getting a test kit if you dont already have one and keep an eye on the water quality, maybe a small daily water change for a couple of weeks would also help.

 

[WILDER]

Your tanks overstocked for  6 gallon. Most you could keep is 6 guppys.Then
they really need at least 12 gallons as there active fish.
Also with the fish expecting babies water quality going to be an issue. As the filter won't
keep up with all the fry as well.
If you want to keep the remaining adult fish you could always upgrade your filter to a 15 to 20 gallon.
 
The flecks on the fins did it look like strands of cotton, cottonwool. Or tiny white spots.