You need to try for at least a 5gallon,10 gallon tank is better. The 2 and 3 gallon tanks just aren't big enough

Your tank sounds very pretty but, Sorry to say the 5 gallon you have is soon going to be over stocked. The rainbow shark will outgrow it fast and depending on what kind of algae eater it is it may also
Check with
www.freecycle.com for tanks and equipment people want to give away and the classified ads in the newspaper. Yardsales are also a good source for cheap aquarium stuff.
I know its hard not having much money but for the safety of your fishes sometimes it's best to just wait and save for what they need.
Carol