That's very helpful. Thankyou for clarifying this and for such a detailed response. I've already gotten a stabilised tank water reading at 7.4. Given that I'm at 6.6 and it's on the edge of being right from an observation standpoint it seems that I'm actually okay and I could indeed push it up to where I feel it needs to be. Just means my drop checker will be glowing in the dark haha.
Any idea why my almost identical tank with no co2 is so low ph wise? I was going to introduce a co2 kit on there too but it's so low now I'd be afraid to lol. Like I say its absolutely identical.