Keeping fish in? That's what I use mine for, then I eat the fish. (and in another thread we've been talking about how fish poo is an excellent fertiliser)

I seem to be the more-or-less resident GH aquarist, at least here on the GH forum, so if I can help, I will be glad to, but first:

How large is the tank?

Naturally, I would tend to encourage fish, but they are living creatures that require care, so if that's not your interest, a covered, lighted aquarium could be a great mini-hot house.

Definitely put it in the greenhouse. If you heat it, the heat will dissipate and help warm your greenhouse. But most importantly, use the water to water your plants. They will love it.

Alternatively, you could set up a bubbler system for the propagation of cuttings.

Add water, place the air stone in the middle attached to pump (for oxygen), heater if need to bring the water temp up; an opaque wrap around the outside helps prevent the growth of algae; float a block of styrofoam on the top; cut 1" holes in the styrofoam to support the cuttings.

Some people add a dash of liquid fertilizer. Others swear by 1-2T of H2O2. If you're feeling experimental, you could try to speed things along with willow water or aspirin (3 tabs/4 gallons, I believe).

Here is a link that might be useful: Bubbler link, one of many