Whether you give your aquarium 100% cleanings or 25% cleanings depends on whether you cycle it (establish a colony of bacteria in the filter to break down wastes). Here is a brief overview of the difference between cycling a tank vs. an uncycled tank:
_______________________________________________ Rotting food and fish waste produces a toxin called ammonia. This is usually what kills fish when the tank isn't cleaned often enough. Given enough ammonia, bacteria called Nitrosomonas start living off it and starting a colony in the tank. They live on all the surfaces, primarily the filter because it contains the most surface area. They eat the ammonia and produce a less-harmful waste product called nitrIte.
When nitrIte builds up, it attracts another kind of bug called Nitrobacter bacteria. They do all the same stuff as Nitrosomonas, only with the nitrIte instead of ammonia. What comes out the other end is a chemical called nitrAte. It's only harmless in big amounts. You then vacuum the nitrAte out of the tank when you do the water change. It doesn't take alot of vacuuming to remove.
In an uncycled tank, you skip the bacteria by sucking all the ammonia up yourself. Since you are cleaning out all the ammonia before the bacteria get a chance to dig in, they never really colonise the tank. So you take care of all the ammonia yourself, which requires bigger water changes to eliminate.
AMMONIA >nitrosomonas bacteria> NITRITE >nitrobacter bacteria> NITRATE => Water Change
So basically you don't have to clean a cycled tank 100% because the remaining nitrate is not very harmful and keeping most of the water in the tank preserves beneficial bacteria. Whether you want to cycle a 2g tank is up to you, but personally I recommend keeping anything under 2.5g uncycled because it is hard to keep the bacterial colony stable in such a small tank. And if the bacteria die off, ammonia will build up in the tank without you knowing it.