To the point, yes tank water is great for floral arrangements. It's already stable and loaded with nutrients plants love. Sadly, water from well-maintained tanks is less effective than water from neglected tanks in theory, as there will be less nitrAte and phosphate built up. I use tank water to fill my aquatic arrangements and it's great. But the elodea I use in those arrangements doesn't exactly have high demands, either and I do feed the water with old dry foods and small scud populations.
Hey, see if you can grow daphnia or scuds in the vase. That would be awesome.