Ocean acidification is much more complex than "less algae and less fish." In fact some algae and bacteria will have giant blooms partly as a result of that, warming, and Nitrogenous runoff. Giant blooms consume oxygen in the primary marine trophic levels, causing die-offs that liberate more nutrients, leading to hypoxic and even large anoxic zones. You also have the concern that acifification can lead to (at first) worse outcomes for organisms depending on carbonates, and eventually (potentially) their extinction.

All of this assumes that we have a pretty solid understanding of what this system looks like as we wrench it from one feedback loop into another, but it's the best we've got.