There’s a lot to unpack here. First the flow that we get from pumps is nothing like the ‘flow’ in the ocean. In the ocean, corals are seeing much more water moving past/through them than even the highest flow tanks. So, while to your eyes it may look like they’re not getting much flow, in reality they are getting orders of magnitude more flow than in our tanks. The other thing to consider is that the ‘flow’ in the ocean is not as directional and laminar as it is in our tanks, there is multidirectional, turbulent, rolling ‘flow’ on natural reefs, so again, to our eyes it may not look like as much flow because it’s not all going in one or two directions. And then on top of the regular multidirectional turbulent flow, there are ocean currents, the tide, upwellings, storms, etc that add to, change, and influence the amount and type of flow corals receive. It’s nearly impossible to replicate in a small, closed box. Even big surge machines used by public aquariums and some larger private tanks don’t come close to matching the amount and type of flow corals see in the wild. The type of flow we can replicate in our tanks is not ideal, the only way we can move enough water over our corals is with pumps that by their very nature, are unable to provide the amount and the kind of flow that corals experience in the wild.
To address the question of whether or not corals need high flow. It obviously varies depending on species, and corals are quite adaptable, but the answer is ‘yes’, they do need high amounts of flow. To properly respirate, corals need quite a bit of flow. Flow also dictates the amount of photosynthesis that can be achieved, pulsing Xenia has the highest rate of respiration and therefore photosynthesis of any known coral, because it pulsing action effectively moves water into and then away from the colony. High flow is also important for moving the byproducts (free radicals) of photosynthesis away from the coral, without adequate flow moving these free radicals away from the coral, they experience oxidative stress that can breakdown proteins and cell membranes and ultimately even kill the coral.
Lastly, high flow is important for keeping detritus suspended so that it can be filtered out, it can be a deterrent for nuisance algae/Cyanobacteria to grow, and it ensures proper oxygenation for all of the aerobic life that lives in your tank. High flow is also important for fish, many fish need high flow to stay fit and healthy. Active fish like and need high flow to swim against, fish from tanks with low flow can be become obese (over feeding is also part of this, but lack of flow is undoubtedly part of the cause in many cases - and like with humans in North America, obesity is far too common in captive reef fish). It can also be considered enrichment, I know my fish seem to enjoy swimming against the flow.
Overall, this just seems like a really bizarre thing to be questioning, as it is, for all intents and purposes, settled science. As I said above, corals are highly adaptable, and so long as they’re getting enough flow to respirate and photosynthesize, they’ll survive, but to truly thrive, they need a good amount of flow (again, what constitutes a ‘good amount’ varies depending on species).
