i don't know either, so i don't use it. I know that there are lines of ferts out there that are invert safe...i would do a quick check at www.planetinverts.com to see.
I dose my 5g shrimp/snail tank w/ dry ferts (mixed into solutions w/ RO/DI) on about a 1/20th EI schedule and do ~40% PWC every other week (sometimes weekly). I add a bit of Excel 4-6 days per week.