Hi
@ChrisD80
Here are some thoughts.
You're drop checker solution doesn't look right to me. I would throw it out and try a new one.
Your PH of 6.5 is pretty low. I'd like to know what you KH is. It could be that your KH and therefore your PH buffer is close to zero, and this may give you an unstable PH. I have some experience with this.
Alternatively it could be that your aquarium soil is formulated to lower your PH?
In either case low PH makes CO2 more difficult. You will notice the above chart supplied by
@steveno only goes down to 6PH.
You will also notice that the lower your KH the larger a PH drop you need to reach 30ppm CO2. Where most people aim for a 1 point PH drop, people like myself with a KH of 1 often need a bigger PH drop, 1.4 in my case to reach 30ppm.
So it's very possible, depending on your KH, that you are not injecting enough CO2.
It's true that most plants prefer temps of 22-24c, but it is possible to grow most in higher temperatures. My tank is never lower than 28.5 and often hotter. If you have a way to keep your aquarium a little cooler I would, but if like me this is impractical, then I wouldn't worry about it too much.
Lastly, plants don't care what your KH is, but your GH matters. My tap ranges from 1-3GH and I found nothing would grow in it. I now add calcium and magnesium and aim for 8GH.
I would says 6-10GH is about perfect for 95% of plants. That said I made an error and ended up with 21GH - plants loved it because I still had a 4:1 Calcium to Magnesium ratio, and they certainly had enough of both.
Let us know what you get for KH and GH readings, and while you're there Ammonia, Nitrite and Nitrate would be extremely useful, and we can go from there.