Hi Carson,
As you already know every tank is different so a lot will eventually be left to your own testing and tweaking. I too started out with the Flourish line and TMG in my 55 gallon. Mixing your own ferts as Rex I believe pointed out is infact a lot more economic, I bought just about all I need now to mix my own for the next few years but I'm still finishing off my Flourish and TMG products that I stocked up on.
I mix a weekly batch of 75ml composed of the following:
15ml Flourish
30ml TMG
10ml Fl. Nitrogen
10ml Fl. Potassium
10ml Fl. Phosphorous
I dose the above mix daily using an Eheim liquidoser and I also add 10ml of TMG and 5ml of Flourish at each 50% water change. Oh yeah and Flourish tabs every couple of months too. TMG contains a lot of potassium however Flourish contains more traces so mixing the two avoided me having to make the choice. Seeing you are using DIY CO2 you can skip the Excel however I was also adding Excel until I upgraded to bottled CO2, why? Because I bought a big bottle of it and didn't want to waste it, perhaps with DIY CO2 it may help a little, who knows? Flourish Trace is a supplement of a couple of the trace elements contained in Flourish that could be used up faster than the rest of the contents. There's a table I put together a little while ago when I was doing the same comparison
http://67.89.4.174/aqua/fert_table.htm From what I was told, Trace might be needed depending on the contents of your tap water and how fast certain elements are consumed in your tank. Another words, if you have it give it a try and see what difference it makes over a month period or so. Nitrogen, Phosphorous and Potassium will depend on your tap water contents, fish load, fish food and amount consumed by your plants so it's difficult to say which ones you will need. As long as you do large weekly water changes you can experiment with larger doses and see how the plants do, just don't overdose the nitrates which as someone else said, only show up as half of the real value on our test kits. Another words after a large water change, test your nitrates, then add Flourish nitrogen and test again, double the difference and you should know how much you have increased the level by. It's a pain at first but once you have a routine that works for you, you don't need to keep testing, just repeat last weeks doses. If the nitrogen calculation becomes a pain, switch to KNO3 which will show up correctly on a nitrate test kit. In the long run you'll probably want to switch to mixing your own stuff.
Regarding your PH/CO2 swings, low surface agitation might explain the high level of CO2 by nightfall, I've crashed my 55 gallon many times with DIY CO2 and low surface agitation combined. Also, your plants may not be absorbing much CO2 at the moment if they are in short supply of any other element they may need. Increase your surface agitation and see if things change.
I'm still playing around with doses and slowly changing to mixing my own but so far the levels above are what pulled me out of the algae mess I got myself into by increasing the lighting and not changing my fertilizer doses.
Hope that helps
Giancarlo Podio