You have a really neat tank!
You probably have a 15 watt bulb over your 25 gallon tank, meaning you have
less than a watt per gallon.
I suspect lighting will be more of an issue for you than temperature. Java
moss might be a good choice. It doesn't need a lot of light, and it's a good
hiding place for fry. It can be kind of messy if you let it float around, so
you might want to tie it to rocks and/or driftwood. Tie it with black sewing
thread in a thin layer -- only one strand deep -- and it will quickly attach
itself.
Willow moss or Christmas moss would also be good, if you can find it. They are
"crisper" than java moss, and I find them less messy. They supposedly like
cool water.
Leigh
http://www.fortunecity.com/lavender/halloween/881/