Help!!

[KeMo]

Ok so I bought a mandarin fish on Tuesday. Looked good when I got it.
I did the drip with him. So after placing in the main tank he hid for about an hour. Then he started to eat pods right away.
Today when I woke up it looks like his back fin is melting away. Looks like fin rot looks bad too.
Could it get that bad over one night ?
So I went to the store that i got it and told them. They tested my water and my PH was abit low around 8.0 . They said that was the problem and to buffer the ph up . To also give him fresh water baths for 1-2 mins until he starts to look better. Is this what i should do ?
It is now about 6 hours since i have came back and it looks worse, It seems to be spreading to his body now.
Please if anyone can help I feel Really bad. My Poor guy.
What should I do ?
I did a 20% water change after I got home and gave him a 1 and a half min fresh water bath ?
Their is one other fish in the tank but it shows no signs of any illness
Oh When i walked in the room tonight I saw a snail like sucking on the end of the poor guy were it looks like its rotting away. He could not get free, I did knock the snail free. It was the snails that live in my sand bed.
Thanks