I had 6 guppys and 1 Catfish Dwarf suckers in my 350 liter tank, one of my guppys are dead, another Guppy is swimming on its back, but killed it because it was suffering, 3 of them are very fat, and only staying at one spot, do they have parasites?

I did put some new plants in a week ago, could the plants have had something on them that make them sick?

The 3 of them there is sick will not eat the peas im giving them, the one there is acting normal is eating it.

I did a salt bath whit epsom salt yesterday and today they are not improve

Also I have another guppy in my quarantine tank, it's acting totally normal and I have feedet as much as the others.

Water:
Kh : 18
GH : 20
Ph 8
Ammonia : 0
Nitrit : 0
Nitrat : 12

Temperature:
23 c

Here is a video i toke of them:

Do they have stringy white poop?

Hmm cant see eny poop from them, ore on the buttom, is that a good sign?

I put alot of peas in to them and thy startet to eat from them, but one of them i swimming on its back, should i then kill it, ore do it still have a chance?