my understanding is that when a currency is fixed, it is just fixed. banks dont buy and sell the currency to keep it at the fixed price, its just that any trading of that currency is done at a specific price and thats it.
Well actually this might be what the central bank does, but implicit in this is the requirement for the central bank to buy and sell currency. In the same way as the RBA might say the cash rate is.... ; they still need to buy and purchas CGS in order to maintain that rate otherwise there would be a disequilibrium in the market.
In the case of fixed currencies, central banks need to either issue or recall currency in order to maintain equilibrium in the FOREX markets. You can imagine what would happen if they didn't; you would have a situation where people who want to see Australian dollars could not do so as there would not be enough buyers (and vice versa).
A central bank can increase the supply of a currency in two ways:
1 - It can hold large local currency reserves and release them or
2 - It can increase the money supply
But when you think about it they are essentially the same thing.
A central bank can decrease the supply of a currency by purchasing that currency of its holders. But to do this the central bank needs foreign currency reserves as they need to purchase the currency in a foreign currency. This is why central banks often do currency swaps with other central banks.