Yeah, I could kick myself. I wasn't sure the sponge filter was working properly but I'd never used one before and I just assumed that was how it was meant to be. The water was smelling a bit too before I discovered it had ammonia in it but I just didn't think. Argh. Still, it's all valuable experience and hopefully there'll be a next time and I'll know not to make the same mistakes again.
My main tank is due a water change tomorrow, and from now on I'm gonna start doing cooler water changes, see if I can encourage them to breed. Any other tips are encouraged.