If the fish are still in a 25l tank, you're going to need to change all the water, at least once a day. Replace the fin rot medication as you change the water. You need to make sure there's no trace of ammonia or nitrite in the water at all.
What they really need (apart from euthanising) is a larger volume of water. If you can't get a bigger tank, a storage box would do. A larger volume of water will take longer to become dangerously polluted.
But I really think you should consider euthanasia. There's no way back for a fish once it's body has started being eaten away like that, I'm sorry to say. We can inform you of the humane way to do that if needs be. Do not, under any circumstances, put them in the freezer or flush them down the toilet.