I had fish too while in University. I in-fact had red crabs too. The problem with bettas are they are too hot headed when placed with others. They look nice and usually kept alone.
At least that's what I heard
I had small tiny colorful fish (forgot their names now but there were loads of them), I had algae eater fish (which was known as fish tank cleaner - it made my tank dirtier then before!!) and I had 2 gold fish with 2 red crabs. I remember I spent more than £100 on everything when bought.
It was very hard to keep them, I was lucky to have friends who were happily willing to clean the tank for me
. Unfortunately all died except the Gold fish which later my mum gave it away because of it's share size (tank was getting small for a big fish!).