I can't contribute to the safety question, but I'll clear up the algae question that Jennewt asked; Flourish excel is sometimes used in concentrated amounts, aimed directly at algae, to kill it off with extra carbon.
Actually, I wonder if the dosing method changes the safety because it wouldn't be as diluted as usual before entering the tank? It would dissipate quickly, once the filter were turned back on, though. Maybe you could remove the axolotls while you treat the algae with concentrated excel, just to play it safe.