Instant White Spot Deaths?

[Marcd]

Hi new here and quite new to fish keeping.
I have a problem with 1 of my tanks where it looks like white spot have instantly killed my guppies?
I changed 10% of my tanks water 2 days ago like i always have done and yesturday noticed my fish started swimming in the top corner of the tank all huddled together.
So thinking this was strange i examined them but no visible problems seemed to be wrong. awoke this morning to find them totaly covered in very large white spots and large spots hanging off their eyes and half my fish were dead and covered in white slime..
Ofcoures something was in there yesturday but can white spot kill this fast?

 

[WILDER]

How many gallons or litres is the tank.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.

When you did a water change could any chemicals gotten into the water.
Any redness to the spots.
Did you add declorinator to the fresh water.

Any signs of darting, erratic swimming, gasping, laboured breathing, flicking and rubbing, fish turning upside down.
Any red streaking on the fish.
What do the gills look like.
Any red sores or tiny red pin prick marks on the fish.

 

[Marcd]

Its a 36litre tank
There was 3male small guppys 2 females 1 loach and 2 old neons which had for about 8mnths and 3 baby swordtails about 5mm long(now just the neons are left)
I changed the water as normal using the same type and make of tap safe that i have always used and use in my big tank.
There was no redness they just had masses of white spots and slime over them.
The only unusual activity was they huddled in the corner together.

I have live plants in this tank which have also turned for the worse and started to look slimy and clumping together.

it looks like it could be some kind of fungal problem thou as to why it came on so fast and killed my fish in 24hrs i dont know.

 

[WILDER]

Your tank was overstocked.
What type of loach is the fish?

Immediate water change and increase aeration.
Were the spots the size of a grain of salt, but they only joined up to for a patch.
Any signs of flicking and rubbing.
Any cotton wool patches on the fish.

Is it possible to load a pic onto the site of the sick fish.
Also need to get water tested. Take a sample of your water to the lfs and ask them to write the readings down for you.