OK, I erred in my last post saying that the fisheri needed copepods. It's their fry that need baby copepods.
That out of the way, either of those should work for a 16g tank but the second one is out of stock so there is no choice at this time.
Be aware too that you need protection over intakes so the very small seahorses can't get caught up.
You shouldn't need a heater, and in fact, may need a chiller.
Tropical seahorses are recommended kept at 68° to 74°F due to their problems with nasty bacteria. These nasty bacteria multiply exponentially, especially above 74°F.
Also because of nasty bacteria, the tanks need to be over-filtered, and if possible, over-sized skimmed with extra due attention during husbandry chores to seek out hidden pockets of detritus/uneaten food pieces that provide food and bedding for the bacteria.
They don't list any specific requirements for the fisheri.
If they get the capensis back in stock, they are a bit larger but temperatures are needed a bit lower.
From their writing:
Tank requirements: Temperature range 68F to 72F. Optimum temperature is 70F. They can tolerate temporary spikes as high as 75F and as low as 62F. Preferable range below 70F. Minimum tank height: 12 inches. No maximum size.