My sterbais and their recurring whisker erosion problem... I am at a complete loss as to what I can do.

(Keep in mind that I live in Canada where I have no choice but overpay for medication for my livestock. ): My options are limited budget wise, but I am figuring this out and should find a solution very shortly. The tank is very, very well established, has FINE sand (not that substrates cause whisker decay but I always have to write that disclaimer...), is always at under 0-0-20ppm, a pH just under neutral, and the fish are mostly repashy, frozen-thawed treats, and high quality pellets. I spent almost 4k total as of now for my fish.)

When I first received my initial batch of sterbais, there was not a whole lot of whisker erosion problems to be overly concerned for but every new cory I would add, pre-quarantined or not, no matter the species, always had some level of initial erosion. I always assumed it may be from differing environments hosting differing colonies and quantities of various bacteria, so as bad as it sounds, I always allowed the fish to adjust without loading them with antibiotics or stressing them further. Also could have been stress from transport compromising their immune system despite prior quarantine procedures taken before my purchase. My albino bronzes bounced right back, so did the peppers, but apparently not the sterbais because the mouth problems would persist. Legs Zeppelin was one of the most badly affected ones and ended up with a really bare mouth despite all medication I had (kanaplex, metroplex, praziquantel). I eventually just accepted her as she was and she did regrow a singular whisker before she passed away from the soap poisoning fiasco (if you know, you know).

The next batch of sterbais was somehow even worse? Again, no problem with the peppers outside of the initial adjustment period. Their whiskers are a little short but they came home with shorter whiskers, so I didn't mind that. The 4 sterbais that came to me were practically bare mouthed again, and once more, it was resistant to the medication I had, the infection just laughed in my face. The only thing that really did halt the infection was a salted methylene blue bath. At that point I started feeling some sense of relief thinking that we might FINALLY have a sense of recovery from this. This lasted around 6-ish months I believe.

Now, semi-recently, the sterbais started having mouth problems for the third time. I was stumped. Kepler, the smallest sterbai, was the most affected with a white, almost completely bare mouth, most of the shoal had bad looking whiskers except for the breeding size female, Elizabeth (and that really is not saying much compared to the long, majestic whiskers the peppers and albino bronzes sport). I remembered something about kanamycin being able to pair well with other select medicines at the same time for medical use, so this time, I did a stock solution of kanaplex, added aquarium salt and methylene blue since it was effective last time, and bathed the ENTIRE shoal for a solid 30 minutes. Everyone remained calm and there was no evidence of poison in the water (which would have made it cloudy, it remained crystal clear), they were just a bunch of angry little men. They were released shortly afterwards and seemed to cope really well afterwards...

... only for TWO AND A HALF WEEKS later, end up with Kepler, another small sterbai, and even my poor Elizabeth, with a mysterious white rash around their mouth and forehead. No fuzziness, just a "white leopard rosette" look around the natural markings paired with irritated oral tissue. Now I am at the point of being extremely frustrated because I have tried EVERYTHING medical with them within my power. Again, my two other species are completely fine and could not be any more comfortable. Whatever the hell is going on is resistant to all my treatments now, and ONLY affects the sterbais.

Please tell me I am not alone... it feels like I am losing an uphill battle with them. One of them floated upside down at the surface until I disturbed them, and I am callous over fishkeeping but they are making me hella nervous at this point. I am currently browsing other medicines and I will purchase a source of nitrofurazone (such as elbagin) and medication that attacks true fungal issues to see if that can help provide them a BIT of relief. I LOVE my sterbais and I am doing everything I can for them, but that stubborn whisker decay problem makes me want to rip my hair OUT. UGH.