Are you sure you didn't mishear your superintendent? 45 ppm just sounds a bit outrageous...
I agree with Noahma, you should look for an imbalance of fertilization before you worry about nitrates in the water.
You may also want to wait on the anacharis and hornwort for a bit. They can often sensitive to chemicals such as the glutaraldehyde in Flourish Excel, among other things - and I know that anacharis doesn't do well with salt. I wouldn't be surprised if it was just the peroxide, but it could be from any number of other reasons, such as temperature or fluctuations in water parameters.. Who knows?
Keeping them out of the situation might save you a bit of trouble while you find out what the problem is. Just a suggestion.
Hope you get it sorted out!