Seahorse died after adding another

I had purchased a kellogg seahorse last week and put him in one of my frag tanks (mostly zoas, blastos, acans...) Horse was doing fine eating frozen brine and pods. It happened that I was at another LFS I frequent that had a Red Dragon seahorse which was a bit larger than my kellogg but I had always heard that they liked company and after asking the LFS if they felt the two would cohabitate and they felt the two should. So after a careful acclimation the RD horse was in the same tank and the two seemed fast friends even seeming to cuddle while sharing the same posts to attach to. Both seemed to eat well immediately after RD was added. This morning the were both gliding around seemingly doing well however this evening I found the kellogg dead in the far corner of the tank. It seemed like his fins were missing and his face was extremely white. Now there are no other fish in this tank whatsoever. Should it be assumed that the RD had a hand in the K demise?

 

I am not an expert on sea horses but using clear thinking I would say he is the culprit.

 

not an expert either by any means, but are there any corals that could have stung it (LPS) or hermit crabs or shrimp that would've nipped the fins after it died?

 

First off, VERY FEW LFS personnel are qualified to give advice on seahorse keeping, although some think they are, while others just outright lie to get a sale.
H. kelloggi seahorses first of all have a very poor reputation for survival and with the time involved, I'd have to believe you couldn't have done anything to make it survive as it was probably a problem when you bought it.
I've never heard of a "RED DRAGON" seahorse so that is a made up name that is not used in the hobby that I've come across in the last ten years.
You should note:
Mixing seahorses from different breeders is a crapshoot as one or both can succumb to pathogens that they are exposed to from the other. Seahorses are known to be poorly tolerant to pathogens they haven't grown up with.
Even doing the 3 drug, 9 week protocol is no guarantee of success.
Also, while some hobbyists do keep seahorses with corals, stinging corals are known to kill seahorses.
Bacteria like vibriosis types are known to be perhaps the biggest cause of death for seahorses in the hobby. While they may be in warm water in their native habitat, in closed systems like we have, the bacteria multiply exponentially with each rising degree, especially above 74°.
The recommendation is that seahorse tanks be kept between 68° and 74° for best chances of success.
Micro life and bacteria are responsible for any missing fin material and for the white portions of the skin. It may have already started before the seahorse died, but one it dies, progression is extremely fast and very visibly seen.
You can check out the links at the bottom of the "My Thoughts..." link in my signature as the links there direct you to the information best known for getting into the seahorse keeping hobby.