I can't remember what the concentration of Excel is. There is a huge thread on APC discussing that kind of thing if you are really interested you should browse through it. Thread: http://www.aquaticplantcentral.com/f...id-all-my.html
I don't think you really need to though, it has already been determined that dosing gluteraldehyde is cheaper than excel (much cheaper). Wet made a calculator program for it that you can find here: http://rota.la/glut/
Gluteraldehyde is more concentrated and easier to over dose with than excel but it will do the same job.
Don't worry about the pH changing. It regularly fluctuates up to a full 1 degree per day in planted tanks, sometimes more. It does not cause issues. What happened with your valve? If you can still control the bubble rate then I don't think it needs to be replaced.