Seahorses tolerate garlic and garlic-based medications very well, so the Garlic Guard in the juvenile tank should be no problem. Those were excellent upgrades to your canister filter and they should certainly help as well.
However, I wouldn’t transfer any of your youngsters from the nursery tank into the new 30-gallon nano cube until it has finished cycling, even with the benefit of those upgrades. The ammonia and nitrite levels you reported would be stressful to the fry and juveniles, and the additional wastes from the seahorses and their food would only exacerbate the problem and increase the ammonia and/or nitrite spikes. Performing water changes to reduce the ammonia and nitrate levels for the benefit of the seahorses you add would only prolong the cycling process.
In addition, I would wait until the full two-week treatment period with Parinox had been completed to minimize the risk that any of the seahorses you transferred to the tank could contract Cryptocaryon irritans. The encysted parasites are very tough to eradicate and you should allow at least two weeks for the medication to eliminate them all before you consider adding fish to the tank regardless of the ammonia or nitrate levels.
On a positive note, those ammonia and nitrite readings indicate that the new aquarium is cycling nicely now, so it shouldn’t be too much longer before the process is complete and the ammonia and nitrite readings drop to zero. Once that happens, of course, you can safely stock the new tank with some of your surplus youngsters.
Yes, sir, I think it’s a very good idea to aerate the 30-gallon nano cube that will be serving as the new home for your juveniles. Cute tanks in general tend to be under circulated, so adding an airstone to provide better surface agitation, improve aeration, oxygenation and offgassing, and promote efficient gas exchange at the air/water interface would be very sensible. For the same reason, it’s advisable to position the spray bar from your canister filter above the surface of the water so that it produces better surface agitation and aeration is well.
As an added benefit, increasing the aeration and the temperature in the aquarium while it is cycling will further accelerate the cycling process. The beneficial nitrifying bacteria (Nitrosomonas and Nitrobacter sp.) that convert ammonia to nitrite and nitrite to nitrate are all aerobic or oxygen-loving microbes, so their population will increase faster if there is a high level of dissolved oxygen in the aquarium. .
In addition to providing extra aeration, I would raise the temperature in your juvenile tank to around 80°F while it cycles. Bacteria multiply faster at warmer temperatures, so raising the water temperature should help stimulate faster growth of the beneficial nitrifying bacteria as well. (Just don’t forget to reduce the aquarium temperature back to normal once it’s finished cycling so that it’s optimal for your seahorses.)
I would gut-load the brine shrimp until your youngsters are accepting frozen foods, which can then be enriched with the Vibrance to provide them with a daily dose of carotenoids, vitamins, and beta-glucan instead of gutloading.
Best of luck getting the new 30-gallon nano cube fully cycled and up and running, Seagazer! Congratulations again on your juveniles and the outstanding success of your nursery set up!