low Ph in newly setup tank

hi i have setup and new tank of 35 gallons specs are as below
Tank size: 30Wx14Dx18H (inches)

Light:T5, 55W x 2

Substrate: ADA Power Sand SPECIAL-M as base and ADA Aqua Soil-NEW AMAZONIA (multi type) 18 litre (tourmoilne BC, bacter 100, and Clear Super is sprayed ,while setting up the substrate)

canister filter( jebro local brand) :with ADA bio rio (2 litre), and NA carbon into it

Co2 pressurized tank and ada co2 checker

plants:Elatina Trianda , and Hemianthus callitrichoides as carpet plant, a piece wood which i guess ferns (may be java ferns i am not sure ) and spike moss attached on it, i bought it because it was looking nice at aquatic shop

currently i am injecting 1 bubble per sec C02 for 8-9 hours with lights on for the same period of time, alternative day 30 to 40 % water change, Adding ADA brighty k , 3 to 5 ml after each water change.

its been 10 days now and i have seen that ferns have turned weak and brownish ,and got some kind of black algae spots on it, there were few spots at the time i bought it, but now its more, spike moss are doing well, Hemianthus callitrichoides, is growing but all the new grown leaves are turning into yellow/ brown color
i just bought a digital ph tester, and found that ph is quite low and counted as 5.3, after changing the water 40% its u to 6.9 and within one hour its 6.2 and than once the lights are on and co2 start injecting , ph gets low to near about 5 , Co2 checker will still shows the there is less Co2 in the water as the solutions shows dark green and its doesn't changes even after few hours, so can some one help me to find the reason for my dying plants, and guide me to balance ph , and Co2 , my tap water counts to ph of 7.1, i have read some where that the new AMAZONIA ADA soil , lowers the Ph, if this is the case , how to settle my ph so i can introduce fishes ,where both plants and fishes live happily , i forget , my current water temperature is 30-31 Celsius