This is a great topic! Thanks so much for bringing it up, and I wish there was sub-forum for these sorts of discussions to be fostered regularly here on R2R.
Given the magnitude of the threat posed by climate change to reefs, one of the most effective things we can do as individuals, or perhaps as an organized collective, is to support organizations and legislation that directly addresses the underlying causes of anthropogenic warming IMO.
Re-planting coral and similar might feel more directly effective (and certainly more interesting and tangible) to those of us who care for these animals in our own homes, but ultimately it's a futile effort if those corals are not engineered with heat-stress tolerant symbiodinium clades. And even that work is futile if efforts to mitigate future warming and acidification of the oceans are unsuccessful.
Some effective ways to give back off the top of my head:
Donate to Cool Earth: This one is what is recommended in Will MacAskill's book "Doing Good Better" as the best climate change charity at the moment. This organization is so effective from an economic perspective that they're able to offset the average American's carbon footprint for roughly $100. To put that into perspective, a Tesla Model 3 @ $40k will lower a single person's carbon footprint by roughly 25% per year of ownership in the best case scenario. That same amount of money given to Cool Earth offsets the entire years carbon footprint of ~400 people.
Creation (and extension) of Marine Protected Areas: Most of the current Marine Protected Areas encompassing coral reef ecosystems don't extend to mesophotic depths. Deep reefs are some of the most biologically diverse and least protected parts of the oceans so advocating for their protection in any way (write, email, call!) is potentially a very effective use of your time if you want to help give back to reefs directly.
Be selective about where you buy your fish and corals. Ask your LFS about what captive-bred fish they carry. Ask about aquacultured corals. Maybe mention you'd be willing to pay a extra for them. In general, providing incentives for responsible business practices given the strain on wild reefs is a really helpful thing. It would be really cool if the reefing community at large could come up with some sort of certification of sustainability that LFS owners and coral/livestock vendors could apply for.
But I think if everyone thought about those choices now and then and sometime went for the choice with less CO2 emissions, that might in the long run help the coral reefs.
