I use bromine and have left for 12 days, returning to perfect water. As close to leaving time as possible I shock (big shock) the water, turn the heat down to 98 and increase the bromine floater a little. Make sure your filters are clean (give them a rinse) so they are not contributing to the petri dish issue.
I'm not sure if turning the heat down helps or not, it seemed like a good idea at the time.
Short of having someone come over and throwing in some MPS, not sure what else you can do, someone else on here will though!