"Thing is, building coral and sea life is super simple [same with killing it]."

That is just simply not true. While it might be helpful to lay down structures for sea life, ocean acidification [1], algal blooms [2], and climate change are all contributing to mass death of sea life, and fast enough where we will see many species go extinct in our lifetime (bluefin tuna will be there soon [3]).

Even related to your own fishing story, check out the fish sizes and diversity in the 1950s compared to now. [4]

[1] https://en.wikipedia.org/wiki/Ocean_acidification

[2] https://en.wikipedia.org/wiki/Algal_bloom

[3] https://news.nationalgeographic.com/news/2006/07/060724-blue...

[4] https://www.npr.org/sections/krulwich/2014/02/05/257046530/b...