What sort of magnitude are the pH changes are we looking at here?
For Killifish? Use peat. You can buy peat pellets which are made specifically for aquaria which can be placed inside filters or left on the bottom of the tank, or use peat as a substrate. If you buy peat which is not intended for use in aquaria (for example meant for gardening), you will need to make sure that it contains no additives (pesticides, fungisides, etc) as these may be deadly to fish.
Buffers are, in my honest opinion, not suitable for use in aquaria. They only work as long as you put enough in and will stop working as soon as they are used up, which is what happened in your case.. the only way you can get around this is by putting a LOT more in. A buffer is essentially a solution of a
weak acid and its conjugate base, or
weak base and its conjugate acid. If you want to know more about it, I would advise that you read about
buffer solutions. Also they can be made up of some quite evil stuff.
My apologies for taking so long to reply! A busy week of family visiting.