Okay, there's a few ways of looking at this.
You are correct in that plants do not need fish products to grow well. There are many amendments available for your use. The problem is that quite a few of those amendments are animal based in origin. Guano, bone meal, blood meal, oyster shell, etc. In order to find a complete recipe that does not contain animal or animal byproducts you will have to seek out a vegan mix.
That said, fish get pulled up on shore by animals and left there by seasonal floods all the time. Fish is also one of the oldest fertilizers in use by humans because it has always worked so well. Where I live in Oregon there is a program where they take spent salmon from the hatchery and deposit them in the forest around the rivers. There used to be far more salmon getting pulled out of the water by bears, eagles, ravens, raccoons, etc and spread throughout the forest. These fish were the basis of a natural seasonal bump in nutrients available to plants in the old growth forests and were a vital part of the overall ecosystem health. Since the start of these fish depositing programs there has been a bump in plant growth and health, which in turn causes better forage and nesting habitat for wildlife.
Alaska Department of Fish and Game: Salmon in the trees
Why trees need salmon, an environmental case study.
Something fishy in the trees: KQED
However, nothing we do to these plants is natural. We have bred away their natural resistance to the onslaughts of the natural world in favor of traits like potency, flavor, and yield. Because of this we must provide a completely artificial environment for most plants. There is nothing natural about potting soil, indoor lights, ventilated greenhouses, pruning, constant watering, denying them pollen, etc. We are not striving for a natural life for our plants, we are doing quite the opposite. We are providing extraordinary circumstances to allow for unnatural and excessive growth of plants otherwise unready to grow on their own. Take the lovely auto flower, which has no THC in it's natural form. We had to breed them excessively to get where we are now.
So, if you're looking for fertilizers that are not animal based because it is an ethical issue for you, I fully support you in that endeavor. Even though I am a hunter that feeds myself and my family on wild meat, I have nothing but support for ethical vegans and vegetarians. However if you are resisting using animal based nutrients because they do not feel natural, I would urge you to reconsider what you consider natural in this process. We use these things in organics, especially in soil building, because they are so effective. Also, a lot of fish and other products are what is considered waste material that would have just ended up in a dump (or dumped at sea) so the animals are seeing more complete use of their whole body instead of just choice parts removed and the rest disposed of.
The choice is yours, and I support that choice wherever you go with it.
Good luck!
