Low KH and PH

so ive been having problems growing plants since i started this hobby. Ive posted here before about plant advice and what not. basically i have 1 main 75g tank
2x 20g tanks. (1) 20g hosts 10 baby angelfish fry about 6-8 months old only other tank is empty. i also put enough ferts/root tabs that is needed from what i was told on the forum. niloc fert/root tab/seachem potassium. my plants almost never die but never grow.

I tested all tanks with API test strip not sure if thats the best but my liquid drop API kits are hard to get consistent tests, need new ones. but i find my 2x(20g) tanks are both the same parameters of course test strips dont give exact numbers but they have
high GH, 0-40KH, PH 8, NITRITE 0, NITRATE 20-80, ammonia 0.

it seems like this used to be my main 75g tanks parameters but now they are
high GH, 0 KH, ph 6-6.5. nitrite 0, nitrate 80-100, ammonia 0.

im pretty stupid i guess i didn't know driftwood lowers PH AND KH even tho it released no brown tannins in my water ever and ive had them all in tank for half a year.i have 1 big piece of driftwood plus 2x smaller logs. is this a sensible conclusion. if it is i know someone here reccomended me to raise my kh/ph to use cuttlefish bone or a buffer type chemical which i have but im a bit nervous changing the parameters... i like the cuttlefish bone method i think more, would i just add it to my tank or canister filter and take it out when tank reaches desired levels? then monitor parameters after/after a water change and add it when i need it? sucks to think ill have to forever add some buffer thing but i do want driftwood.
i also am working on getting my high nitrate down, came from a mistake with over liquid fertilizing months ago, water changes dont seem to help though. i might look at some other ways.
real sorry for the long post...

For what it's worth, the driftwood is not lowering your pH if it's not releasing tannins but it is absorbing your KH which is allowing the pH to fall from the acids being produced by your fish and the nitrates since there is nothing stopping it. There are ways of adding back KH ( i.e. adding epsom salt, crushed coral or shells, etc) but you do not want to change this drastically as it may kill off your plants.
What I would do is get some better test results for your KH and pH of your source water and if the pH is higher than 6.5 and your KH is higher than the 40ppm, do more frequent water changes to get your pH and KH up and your nitrates down. Depending on what plants you have, the " ideal" pH for most plants is between 6.8 and 7.5 so you don't seem to have far to go. It's always best to do changes naturally instead of artificially ( through additives).
If your source water is not going to get you where you need to go, we'll have to go another arduous route.

Hope this helps.