It is also possible that the temperature is too high for them. Aside from the often repeated bacterial risks at temperatures higher than 74 degrees, high temperatures can also make your seahorses lethargic. Luckily, those max tanks are built to accomodate a chiller.
Also, I looked through the specs, and depending on how the flow is set up in these things, you could be fine, or the flow could be too much. You could try fitting spraybars to the powerhead outlets (if possible?).