I didn’t realize you were concerned about how well your Zulus were doing in your seahorse tank. If their activity level is down, and they’re not eating as well, or you are simply worried that they’re not thriving in your current seahorse setup, I can understand why you might want to transfer them to clean, freshly mixed seawater in your nursery tank instead. Small as it is, they would have the whole 5 gallon tank to themselves, so I can see what you might think they would do better on their own in freshly mixed saltwater.
That is certainly your prerogative, Nigel, and if you want to try it, that’s fine. Be advised, however, but you’ll have to monitor the water quality — especially the ammonia and nitrite levels — very closely in your nursery tank all the while they are there, and that daily water changes will be required in order to maintain decent water quality. You’ll also have to provide the five-gallon nursery tank with plenty of plants and enough holdfasts to offer the seahorses suitable hitching posts and a sense of security. And it’s vital that you maintain the temperature in the nursery tank below 75°F at all times for your Zulu’s. If you want to try that for 1-3 weeks until your male gives birth, proceed with caution and be very diligent about maintaining the water quality.
While you are waiting for your pregnant male to give birth in the nursery tank, which will be doing double duty as a paternity tank in your case, you can fix up your main seahorse tank so that it’s ready for the return of the Zulus afterwards. In most cases, the surest way to improve your water quality and correct the water chemistry is to combine a 25%-50% water change with a thorough aquarium clean up. Siphon around the base of your rockwork and decorations, vacuum the top 1/2 inch of the sand or gravel, rinse or replace your prefilter, and administer a general system cleaning. The idea is to remove any accumulated excess organic material in the sand/gravel bed, top of the filter, or tank that could degrade your water quality, serve as a breeding ground for bacteria or a reservoir for disease, or otherwise be stressing your seahorses. [Note: when cleaning the filter, your goal is to remove excess organic wastes WITHOUT disturbing the balance of the nitrifying bacteria. Do not dismantle the entire filter, overhaul your entire filter system in one fell swoop, or clean your primary filtration system too zealously or you may impair your biological filtration.]
At first glance your aquarium parameters in your seahorse tank may look great, but there are some water quality issues that are difficult to detect with standard tests, such as a decrease in dissolved 02, transitory ammonia/nitrite spikes following a heavy feeding, pH drift, or the gradual accumulation of detritus. A water change and cleanup is a simple preventative measure that can help defuse those kinds of hidden factors before they become a problem and stress out your seahorses. These simple measures may restore your water quality and correct the source of the stress before your seahorse becomes seriously ill and requires treatment.
Add a good grade of activated carbon that’s low in ash and free of phosphates to your filter, and replace the used carbon with fresh, clean activated carbon on a regular basis. (If you don’t replace the activated carbon regularly, it can leach back any undesirable substances it has removed into the aquarium after it has reached its capacity.) Carbon is activated two ways, either with steam or with phosphoric acid. The type of carbon that is activated with phosphoric acid contains phosphates, which can be leached back into the aquarium water and promote the growth of nuisance algae. So you will want to avoid that type of of activated carbon. The carton or box that the activated carbon comes in should be clearly labeled and state specifically that it is "steam activated" or "phosphate free" or something to that effect if it’s a suitable brand for your aquarium. Adding Chemi-Pure, a Poly-Filter pad, or a good brand of activated carbon to your filter will provide your seahorse tank with chemical filtration and help to maintain the water quality while removing any residual copper sulfate.
Heat stress is the number one reason that Zulu-lulus (Hippocampus capensis) fail to thrive in any given aquarium, so make sure the temperature in your seahorse tank remain stable between 72°F-75°F at all times. And if you have any concerns about ick or parasite problems in your seahorse tank, consider reducing the salinity or specific gravity to between 1.010-1.016 rather than adding copper.
You needn’t be concerned that your filter will "eat" your seahorse fry if you take the proper precautions. Be sure to screen off the intake tube. This can easily be accomplished by adding a filter basket to the intake tube or by mating it to a sponge filter.
Best of luck with your Zulus and their progeny, Nigel!