Ordinarily, it’s never a good idea to attempt to keep seahorses with different temperature requirements together, but in the case of Zulus (Hippocampus capensis) and Mustangs (H. erectus) that’s actually quite a workable arrangement because both species of domesticated seahorses would thrive at a stable aquarium temperature of 72°F.
You’re absolutely correct in that the Zulus like somewhat cooler temperatures, and it’s very important for anyone who keeps H. capensis to make sure that their water temperature never spikes above 75°F. Zulu-lulus do well at temperatures between 68°F-75°F, and a temperature of around 72°F should work very well in your case, Haynes.
That would mean you need to drop your water temperature several degrees from the current reading of 76°F. There are some very affordable mini aquarium chillers but they are designed for smaller aquaria and wouldn’t make much of a dent in the water temperature on an aquarium above 20 gallons. For example, the CoolWorks Ice Probe and Microchiller units are ideal for small tanks (10-15 gallons or less) and will drop the water temperature up to 6-8°F below the ambient room temperature in nano tanks:
Click here: CoolWorks Ice Probe with Power Supply – Marine Depot – Marine and Reef Aquarium Super Store
Click here: CoolWorks Microchiller – Marine Depot – Marine and Reef Aquarium Super Store
Depending on how large your Mustang tank is, I’m not at all sure they would have the desired effect in your case, Haynes. If not, then you’re probably looking at a chiller that would cost a few hundred dollars instead.
But if you can choose to stable aquarium temperature of around 72°F, then it certainly would be possible to keep Zulus and Mustangs together, and Zulu fry are indeed among the easiest of all seahorses to raise.
Best of luck with your seahorses, sir!