FWIW, I've NEVER checked nitrates as they won't cause my seahorse deaths, especially with proper sized/timed water changes.
I don't think 3 shrimp a day per seahorse will be sufficient for long term survival so be extra vigilant watching to see if they begin to thin out in the main body.
While Brazil does have some erectus, it has been in my experience that most coming from brazil are H. reidi. ID from pictures is difficult but you can go to the ID Guide and check out for yourself.
https://cites.unia.es/cites/file.php...-seahorses.pdf