Agree with the others, increase gravel-clean-water-changes and this is one time when it'll be -good- to have a healthy section of carbon in your filter.
just scanned the thread but if you've got sand substrate you get a bit of a smell when you mix it over as gas gets trapped within it . as long as you do it regullally the smell isnt to bad, leave it to long and the gas will most likely harm the fish.