I have a 20 gallon planet tank that has been running for about 8 weeks. It is not a dirted tank. I have water wisteria, an Amazon sword(there another by the right corner but i don't know its name), Mexican oak leaf,rolta india, ludwigia narrow leaf, green cabomba, anubias nana, blyxa japonica and another species i dont know what it is (i used to had rotala macrandra but it all died off). This is my first planted fish tank and now it is not doing well. I had tons of oak leaf in my tank but now only weak steams with few leaves remain and once the leaves are gone form the steam they don't grow back, only another weak steam. I got my water wisteria 3 weeks ago and now it just like the oak leaf. I have yoyos and they have been digging them out. I got the green cabomba on my first week and it used to be a nice green color and a lot of leaves but now it lost like 50% of them and now its like a yellowish green color. I got my rolta india on the 1st week too its growing realy good but when i cut the steams nothing grows directly on the chopped area but near by on the side and its so yellow that's it almost at a whitish color. The ludwigia narrow leaf lost a lot of leaves on the bottom. I use this for my plants: Flowinaxis Carbon Source, Flourish Nitrogen and Potassium( i got these 2 weeks ago) and Total Aquarium Plant Nutrient Supplement. I use Prime for new water. Also now my PH is around a 7 but i used PH Down to lower it. I have an acid buffer for PH but i have no idea how to use it. What should i do?

This is my tank. The picture is a bit green shifted.