The pH sounds just right to me. If your tap water turns out to be 8.4 after sitting out, then I don't see a problem.
MANY plants live and thrive in hard water. I can only think of a few that NEED soft water inorder to thrive.
Having a tank with a pH of 8.4 and driving in down to 7.4 with CO2
, sounds just about right in my mind.
I think you are hunting down the wrong issue here.
How old are your light bulbs?
What are you dosing now?
Do you have a CO2
What is your BPS (bubbles per second) of the CO2
What are the parameters of your aquarium?
Type of CO2
It sounds as if you aren't injecting nearly enough CO2
. Having a wet/dry sump doesn't help this out AT ALL. It will off gas a lot of the CO2
, so you need to be vigilant in maintaining 30 PPM CO2