Is there any danger breathing the fumes?
I've never used it but Duane (trido) has posted about it before. So, here is a cut and paste, and link to the caution he gave the last time I asked about it
http://www.reeffrontiers.com/thread...-clean-skimmer?p=710744&viewfull=1#post710744Wear safety goggles and don't breathe the vapors straight from the bottle. If you spill it straight onto your skin it can or will burn you. I used it a lot doing brick work as a job and have had it on my skin on many many occasions and a water rinse right away otherwise it will leave a burning stinging sensation. I use it because I get tired of using a gallon of vinegar at a time, and then waiting for such a slow process to happen. Muriatic acid works very fast if you make a strong solution. As little as 15 minutes for heavily encrusted equipment.
It can be dangerous for your skin and respiratory system if care isn't used with it.