night, so need the code to recognise this and cease from 7pm to 7am.
Night is not necessarily 7 pm to 7 am. Depending on the latitude and time of year, that's not even necessarily a close approximation.
Just (fish) food for thought. A light sensor might make sense as part of your control system.
which works fine as a clock.
What else do you need it to work as?
Ordered a couple of DS1307 RTC's just to cover my bases (not yet arrived).
Why, since you already have a working clock?
Is it possible to change the specs of the TimeAlarm.h library to include 36 odd alarms per day
Yes, but why? If you know that you want to feed the fish every 20 minutes, does it matter if it is at 7:13, 7::33, and 7:53 vs. 7:00, 7:20, and 7:40? Either way, you can feed them whenever you like. The even numbers are conceptually easier to work with (minutes() %20 == 0), but that can be (minutes() % 20 == 13) just as easily.
Is it possible to 'start' an alarm 'window' at 7am and 'end' the alarm window at 7pm everyday?
Yes, but again, why? The clock knows whether it is AM or PM. In the feedFish() routine, check to see if it is AM (and feed the fish) or PM (and do nothing).
What are your thoughts on doing it this way?
I wouldn't. But, then, I know how to use a clock.