Sudden Guppy Diseases & Death

Hello people,

I've just gotten 2 female guppies from Yun Feng Quarium yesterday, acclimated them into my 20G planted tank yesterday & woke up to 1 dead guppy & 1 diseased guppy with symptoms of fin rot & some white stuff taken over part of its body, probably fungus attack.

I'm using ADA aquasoil & getting a pH reading of 6-6.5. I'm wondering if this is the cause of the deaths.

I also have another 7G cube with ADA soil & pH 6-6.5 with another female showing signs of lethargy...

Anything I can do to help them? Perhaps a salt bath? Is ADA soil bad for guppies? I heard guppies require neutral-alkaline water & hard water. But ADA softens water and makes it acidic... Can I add coral chips or shells to the filter to raise the hardness & pH?