used this method today and afterwards noticed very strong smell coming from tank. kind of reminded me of diy co2 solution. not pleasant.
hair algae is dying off and turning white. smaller patches of BBA are turning red. no dead inhabitants. still too early to see full results.
i used 6 tbsp in about ~20g of water. 30 mL of Excel after.
I hope everything turns out ok....that's too much Excel I believe (5mL/10gal target charge should've been 10mL in your 20gal tank...assuming you have 20gal of water); adding 3x the Excel may be the odor you're smelling....does it smell just like the bottle of Excel ? I wouldn't know as I use straight glutaraldehyde (has that sweetish pungent aldehyde odor)