That’s a shame! I’m very sorry to hear about what happened to Harry. Please accept all my condolences on your loss.
If he was breathing heavily and there were white dots left behind in the dipping container after you performed the freshwater dip, it does sound like Harry was ailing and that the most likely suspect may have been ectoparasites that attack the skin and gills of the fish. If that’s the case, Marilyn, you will want to wait a good 4-6 weeks before you consider finding a new mate for Bernice. That will assure that she is not going to be affected by the same affliction that led to Harry’s demise. Virtually all ectoparasites or protozoan parasites will have completed their full lifecycle within a 4-6 week period, so if Bernice were going to be affected, she should be showing signs of a problem within that 4-6 week time span. If she is still thriving after that length of time, then you’ll know that it’s safe to introduce a new male to the aquarium.
If you are concerned about Bernice becoming lonely by herself in the meantime, Marilyn, you might consider taping a mirror up against the aquarium glass where she can get a good look at herself. Seahorses will often interact with their own reflections in the aquarium glass, so having a mirror-image seahorse that moves in response to her own actions can be very reassuring for a solo seahorse and perk up the isolated individual dramatically. It’s an effective technique for a situation like yours and can fool the lonely seahorse into thinking he or she is still in the presence of other seahorses.
Freshwater dips are normally very safe procedures for seahorses, providing they are done properly. I know how conscientious you are, Marilyn, so I’m quite certain that you followed the instructions to the letter, but for the benefit of other hobbyists reading this post I will point out that it’s important to adjust the pH and water temperature of the freshwater so that it’s the same as the pH and temperature of your seahorse tank before you perform the freshwater dip. This is especially crucial when you are using reverse osmosis water, which is ultrapure and very soft, which means the pH of the RO water is considerably lower than the pH of a marine aquarium. If you do not match the pH in the dipping container with the pH of the aquarium, there is a risk that the seahorse will experience pH shock during the freshwater dip, which can sometimes be fatal.
Please let me know if Bernice develops any of the symptoms that Harry was exhibiting, Marilyn, such as heavy breathing, loss of appetite accompanied by weak snick, and rarely venturing from the bottom. There are more effective treatment options for weak snick or an outbreak of ectoparasites then freshwater dips, which are really just a first aid measure that can provide them with some quick relief. If Bernice starts to show any of the same signs of a problem, please contact me right away and I will help you determine an appropriate treatment that may produce better results for you.
Best of luck with Bernice and with finding her a new mate, when the time comes, Marilyn.