My actual question is
this - can I hold the fish in a fishbowl or something while I put down a
plant-friendly substrate?

Don't bother with a plant-friendly substrate. It won't do you any good unless
you upgrade the lighting on your tank. (FWIW, the standard tank hood does not
provide anywhere near the light most plants need.)

I suspect that what you have is a 29 gallon tank with a 20 watt bulb over it.
I had a tank like that for years, and only very low-light plants would grow it.
Not only is the lighting very low, but a 29 gallon tank is tall for its size,
meaning the lights are farther away from the plants.

If you are willing to upgrade your lighting, yes, you can put the fish in
another container while you change substrates. (I recommend Seachem's
Flourite. Three bags will be just right.) I would also remove the UGF. Use a
power filter instead. (I have an Aquaclear 300 on mine.)

I did that when I converted my 29 gallon tank into a planted tank. I bought a
15 gallon plastic sweater bin from K-Mart for $2.99. I filled it half with
water from the tank, and half with declorinated tap water. I put all the fish
and decorations into the bin, then scooped all the gravel from the tank, pulled
out the UGF, and put in the new gravel. It took two or three hours (mainly
because I rinsed the Flourite right in the tank). Then I put all the fish back
in the tank. (Because of the Aquaclear power filter, I didn't have to cycle
the tank again. You should do something similar: let a power filter run on
your tank for a couple of weeks before removing the gravel. And do not clean
or change the media at the same time you change the gravel.)

3) Assuming we can save the fish, what sort of Florida native aquatics
should I keep an eye out for? I'd like to keep to low-light levels, low
tech

It is possible to keep low-light plants in a tank like yours. I kept java fern
and anubias for years. They grew slowly, but they grew. However, your plant
choice will be severely limited unless you upgrade your lighting. You may not
find many native plants that will adapt to such low light. And there's really
no need to change your gravel if you're going low-light. Low-light plants
generally don't need a rich substrate. Most of them, like java fern, java
moss, Bolbitis, and Anubias, actually attach to rocks and driftwood rather than
rooting as normal plants do.


Leigh

http://www.fortunecity.com/lavender/halloween/881/