I have a sulphur denitrator that I am very happy with. But, it will only work if the tank's bioload isn't too high. Don't waste your $$$ if you want to use it on a heavily populated fish only tank.

Also, if your NO3 levels are above 20 meq/L, then you'll need to do old fashioned water changes to get them down, so the no3 filter can handle it.

That said, yes they will work in most reef applications. Just don't expect miracles.