You should be able to plug your low voltage transformer into a standard UPS, that should be fine. I have all my "critical" loads on computer type UPSes since I have these in abundance.
If you want to get fancy, read DIY, here are a few other suggestions:
Depending on the voltage needed, a CCTV camera power supply can be used for lighting in many cases (usually available in 12v or 24v DC
) - many of these have the option to add a battery backup internally within the PS (12V7AH type battery like used in alarm systems). These power supplies are usually sold to power cameras in multiples of 4 (4 channel, 8 channel, 16 channel) in varying amperages (1amp, 2.5amp, 5amp) so you can power more than one device. I have used these transformers (power supplies) for low voltage LED
lighting in stereo cabinets, under kitchen cabinets and counters, etc. I can't see why not fish tank lighting if already low voltage.
If you want to use a transformer with a timer, look at Intermatic (Malibu) lighting timers for low voltage landscape lighting.
Hope this helps