Plant dying- accumulation of problems

I have two plants I treat exactly the same, the one I planted 2 weeks and 5 days ago has problems the one I planted 3 weeks and 4 days ago is fine. The bad plant has the sides of its leaves curled up since the day it was born and last week its leaves have been getting yellow and dry.
I have fed them onely one time, about a week ago. There is sun about 12 hours a day but about half of the day is kind of cloudy.
I always add acid to the water I use because here water has a ph of aprox. 8.
I water them every 4-3 days.
I think the ph might be too low (not exactly sure what is the ph since I use a pool ph tester which doesn't go bellow 6.8, however the ph is way below that). However the side of the leaves have always been curled upwards even before I used the acid on my plants.
2nd attachment is the good plant 3rd attachment the bad one.

Some plants are just not as healthy as others. Its not that uncommon for one to have issues and not the other. They are too little for fertilizer you should wait till they are 5-6 weeks old and only give them a 1/4 strenght till 7-8 weeks then 1/2 strenght. There has to be something better then acid you can use to lower your ph. It cant be very healthy for the plants. Im sure someone will tell you someyhing else you can use. I put Pond Water Detoxifier in my tap water to remove the bad stuff like chlorine, chlorimines and heavy metals. All that crap retards growth. If you have access to the detoxifier or a chlorine remover you should give it a try your plants wil lappreciate it! Good Luck!

I think I'll put the plant on new dirt. The acid I'm using is one I bought from ace home center specialy for plants, I'm not sure if it's for any kind of plant. Do you know if I can use something organic and not so strong, like coffee beans or lime?

Hi psangangs,
if i corretly undestand the healthy plant is 3 weeks old ?
In my opinion is too small to be 3 weeks old. Looks to me that your dirt is TOO compact and maybe roots are having trouble to oxigenate themselves.
So my suggestion is to transplant in new dirt but before do this add LOT of perlite in the dirt. Perlite is really important to mantaine the dirt not compact. Roots will love it and maybe also the other plant will enjoy it. Also if your dirt is pre fertilize don't add anything yet because they are still too young. Wait another week imo

Peace and good luck.