According to the Jawbone Hyposthesis (Matsunaga & Rahman 1998), seahorses have evolved in such a way they lack the typical GALT (Gut-associated Lymphoid Tissue) found in vertebrates.
Presumably this is because of their feeding habits where they primary eat live foods out of the water column and pretty much graze throughout the day. Their immune system is not designed to eat dead or decaying foods.
They also have a very short digestive tract that is almost a straight tube. While it works for them, it is not the most efficient system.
They don’t store much in the way of reserves and food moves quickly through their digestive tract, thus the constant grazing.
Prior to seahorses being added to CITES (May 2004), many, if not most hobbyists obtained wild caught specimens, and they were often, if not primarily, fed live foods. Rarely did anyone worry about temperatures. Many were able to keep seahorses at warmer temperatures without issue. Since the advent of Captive Bred seahorses that primarily eat frozen foods, a trend has been noticed and associated with temperatures and long term survival in home aquaria.
Granted, the evidence is anecdotal.
As a breeder, retailer, and one who answers calls on a help desk daily, with customers both participating and nonparticipating in message boards, I have noticed that with few exceptions, hobbyists’ have difficulty keeping seahorses long term (18 months or longer) at temperatures above 78 degrees. Those who have kept them longer, in some cases 5 to 10 years, almost universally keep their temperatures below 75 degrees.
Our own in-house experiments find similar results.
Those that do succeed at warmer temperatures, I have noticed, primarily feed live foods and/or have a much higher flow rate (greater than 15 turns/hour).
I suspect a couple of things come into play.
First, the typical hobbyist feeds their seahorses once or twice a day. This is an unnatural feeding pattern as seahorses are designed to opportunistically feed out of the water column as food passes by.
Next, some of the food being fed into the tank ends up settling on the substrate due to low flow rates and/or overfeeding. It then becomes colonized with bacteria and protozoa. When the seahorse becomes hungry and if there is a lack of food in the water column or becomes trained to, begins to forage for leftover food on the bottom. This in turn can lead to issues with both bacteria ingestion and protozoa loading of the snout and gills leading to weak snick and/or rapid breathing.
Hobbyists then compound the issue with excessive nutrient loading by having too small of a tank, overstocking, filtration schemes that are not robust enough and improper feeding techniques or food.
A typical pair of seahorses can easily consume a cube of frozen mysis per feeding 3 or more times a day. This is a lot of nutrients.
Bacteria, both good and bad are ubiquitous to our systems.
In systems with high nutrient loads, they will proliferate. Most of the time the beneficial bacteria wins out and what is considered pathogenic bacteria remain at bay until conditions allow them to become opportunistic. Vibrios of course fall within this category.
Not all varieties of vibrio are bad, but some have been shown to increase their virulence at warmer temperatures.
Given this scenario, it is pretty easy for the gut of the seahorse to become populated with undesirable bacteria. Improper setups, tankmates, husbandry etc. can further exacerbate the issue with stress and then of course, problems are encountered.
I mentioned with the temperature with few exceptions. Virtually every case I have encountered where the hobbyist has succeeded at warmer temperatures (above 78 degrees) they have had a higher flow rate or feed primarily live foods, namely live shrimp. The higher flow rates do a better job of removing uneaten food, in general are a healthier system and from a casual observation, most of these hobbyist have a more robust filtration scheme.
Seahorses, despite what many believe, can handle a fairly high flow rate fine. The feeding of live foods has the obvious advantage of not going through the decomposing process and I suspect to a limited extent, provide some beneficial flora for the gut of the seahorse much like probiotics are used in aquaculture gut loaded into live foods.

This is a link to Dan's site.

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