If you just bury fish carcasses in the garden, the effects will be the same. Do you own a cat? You will!
Do you have any wild or ferral animals living nearby? They are gonna love you:-)

You can add fish in small amounts to a hot compost system. Smash them up first for faster decomposition.
Making your own fish fert involves having fish rot in a sealed container for a few weeks with some sawdust added. Not sure of the details but it can be done. Pretty much any fish will be fine.
(It really is lots simpler just to buy the finished product.)

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The secret is the soil.