I can't help with the seahorses, but I have had pipefish and mandarins. You'll be hard pressed to keep the copepod population high enough in a 15g tank to support even one of either of those fish. While my mandarin will eat some frozen (when she feels like it
) and black worms, she mainly hunts copepods nonstop. My pipefish never even looked at frozen. I had to hatch baby brine shrimp for them daily to keep them fed. There are some good threads for baby brine shrimp feeders and pod farms, if you're interested I'll hunt up the links for you. I had to rehome the pipefish, they were too much work. Hatching baby brine shrimp daily just wasn't for me.