Is water flow an issue? I keep an SPS dominant mixed reef tank with lots of live rock to provide shelter from water flow. I am of the understanding that too much flow is bad for horses pipefish.
Many websites on Seahorse care copy very old information. When it comes to flow with seahorses no flow or extremely low flow is bad as well as constant ripping flow. In nature seahorses are commonly found in high flow areas. In nature the flow varies with the tides. so they need places to hide from it as well as periods of low flow.
I would suggest flow of 10-20x tank volume and places to hide as well as breaks during the day where the flow is 5-10x.
The reason is you want the flow to keep the frozen mysis shrimp suspended in the water column. If you feed heavily 2x a day, much of the food will start to rot on the bottom or in the rocks. If the seahorse consumes this they will get an internal infection. Seahorses have a very primative digestive system and are unike other fishes. They can not tolerate this and it will kill them.
This is another good reason to keep the water much cooler. 72F is the MAX recommended temperature. H. Erectus are native on the Atlantic side of the USA all the way up to and past New York City. So they are NOT a tropical species and need cooler water.
If you are trying to keep seahorses in an SPS tank, it will be challenging. I keep SPS with my seahorses. You will have issues.
First you will be growing SPS at the bottom edge of their tolerance for temps. I have found most SPS will survive and grow (slowly) at 70F-72F
Next, seahorses are messy eaters and water quality is a challenge. I suggest oversized filtration and even Algae turf scrubbers. You will be using GFO on a regular basis.
Seahorses are not fans of bright lights. You will need areas of shelter during the peak of lighting. In nature SH are frequently found under bridges, docks and other shaded areas. Keep your intense photoperiod shorter and then a longer very low light periods so you can view them.
And as I mentioned limit your flow.
With SPS High nutrients needs high light and high Alkalinity and higher flow. This will be impossible. You will to high nutrient, low flow and lower light.
as with most thing related to seahorses. These recommendations will change over the years as more people push the limits and learn what are the perfect conditions that allow seahorse to live longer in captivity. 20 years ago, seahorses lived 3 years in captivity. Today's recommendations has increased this to 7 years on average