Is Fish frenzie of any benefit?
 

My seahorses have only ever eatten Hikari or San Francisco Bay mysis. I tried to vary their diet with PE mysis or krill but they wouldn't touch it. Recently I tried mixing a small amount of fish frenzie in with their mysis and to my surprise they ate it enthusiastically. Fish frenzie can offer a varied diet and also contains a probiotic. Do you think it is a good idea to add a little? Mysis is still my staple but it seems a little could be a good idea.

I'm not familiar with that product, but in general I would have to say that if they eat it or it helps promote a feeding response in your horses then IMO that gives a good reason to continue. Ive never had a CB horse that wouldn't aggressively devour PE Mysis. The only time I've run into issues with PE is that the freshwater mysid in it tend to be quite a bit larger than Hikari and some smaller horses couldn't snick a whole one but they would usually take the tail of and leave the heads. I've never tried krill, and I would think that they are too large except for maybe the larger species like h. Ingens or abdominalis, neither of which I have kept.
Just curious but is there any reason why you were trying to switch to PE?
Another thing you might try is Selcon, sometimes that make the food more attractive and it also adds nutrients.

IMO, using Selcon/Selco on frozen foods is a waste of money and a sure bet to degrade water quality over time.
As the emulsions don't soak much into already wet frozen foods, it's for the most part, only coating the mysis, and, when added to the tank, this coating quickly comes off and into the water column thereby adding fuel to the fire to feed the nasty bacteria.
I actually rinse off PE mysis to remove any residual enrichment that could get into the tank.
Now if you want to enrich LIVE foods, then you have a whole different story. You can actually tailor make an enrichment to provide specific things to your seahorses and seahorsesource.com has multiple items you can use, or, you can just buy Dan's Feed, already mixed.

I should give a update on the fish frenzie and my seahorses. They ate it enthusiastically mixed in their mysis for about 3 days and then began to get persnickity. They would pick out the mysis and spit out the fish frenzie. I eventually gave up as I did not want uneatten food to degrade my water quality.
And you are correct HorseoftheSea in your guess that 3 of my 4 ponies are younger and smaller and that is probably why they prefer smaller mysis.