There seems to be some misunderstanding about the tax-fixed (Cost per epoch) and tax-ratio (Pool margin). For that reason an explanation (clarification):
tax-fixed (Cost per epoch): this is the fixed cut the stake pool will take from the total reward due to the stake pool;
tax-ratio (Pool margin): this is the percentage of the remaining value that will be taken from the total due
The fees taken by the pool are to get out of the costs of running a pool and of course to also earn a bit.
Example of a pool having a tax-fixed (Cost per epoch) of 100 and a tax-ratio (Pool margin) of 4%:
Let's say the pool produced blocks and the total rewards are 1000. Then the pool will take 100 (Cost per epoch). This means 900 remaining. Of that 900 remaining the pool takes 4%, which is 36. The remaining will go to the delegaters, which is 1000 - 100 - 36 = 864.
The Cost per epoch is not paid per delegater, but is a fixed fee the pool takes to get out of costs and is taken from the total rewards.
Hopefully explanation above supplies some clarity.
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The Cardano (ADA) network will perform a scheduled upgrade on Feb. 20 to introduce Ouroboros BFT, an improved consensus mechanism. The update is an intermediate step between the current Byron era and Shelley, which will introduce staking on mainnet.