Flashing Guppies

[Fish'd]

I wish I never had to post here.

After I put down my previously sick yellow guppy (the alcohol trick didn't work like it was supposed to, so the whole experience was terrible), I thought of taking all of my guppies to my art teacher tomorrow, as none were sick.

Now they're all flashing.

I assume this means parasites, so I bought Jungle tablets. I haven't used them yet.

Is it parasites or just the overstocking? I want to give these guppies up as soon as possible.
If I treat with the tablets, that could take several weeks.

What is it, and what do I do? I'm getting fed up.

Thanks in advance.

EDIT: I did a large water change right after the guppy died.

 

[mosaicguppy]

Did you test your water? Do you see any spots on them?

 

[Fish'd]

I don't see anything on them. I haven't tested the water and can't till the evening.

Thanks for responding. <3