It sounds like you have done an excellent job of addressing some of the most common causes for a loss of appetite in seahorses and pipefish, PonyGirl, such as deteriorating water quality, low levels of dissolved oxygen in the aquarium water, internal parasites such as intestinal flagellates, and tempting the pipefish with choice live foods, all to no avail.
You have done very well in that regard thus far, but do have a couple of other suggestions to offer that may help, PonyGirl.
First of all, I would continue to work on your water quality by performing a series of partial water changes using freshly mixed saltwater that has been pre-adjusted to the same specific gravity, temperature, and pH as the aquarium water. Regardless of how your water chemistry appears right now, some water quality problems never show up on standard test kits or are transitory in nature and difficult to detect. Go ahead and perform a series of 25%-35% water changes immediately to safeguard the water quality and replenish depleted trace elements and minerals, PonyGirl. (At first glance your aquarium parameters may look great, but, as I mentioned, there are some water quality issues that are difficult to detect with standard tests, such as a decrease in dissolved 02, transitory ammonia/nitrite spikes following a heavy feeding, pH drift, a deficiency in trace elements/minerals, or the gradual accumulation of detritus and organic loading. A water change and cleanup is a simple preventative measure that can help defuse those kinds of hidden factors before they become a problem and stress out your seahorses. These simple measures may restore your water quality as well as your pipefish’s appetite.)
Next, try encouraging the pipefish to eat by using some of the appetite stimulants that are known to help break under strikes with seahorses. For instance, get some Seachem Garlic Guard from one of your local fish stores and begin using it with your live foods or frozen Mysis right away, PonyGirl. The Garlic Guard acts as an appetite stimulant so adding it to the live foods or two frozen Mysis may trigger a feeding response from your pipefish. Call around to the pet shops and fish stores in your area and you should be a will to find one that carries products from Seachem Laboratories, including the Seachem Garlic Guard.
If you can’t find the Garlic Guard by SeaChem locally, PonyGirl, home hobbyists tell me that a bottle of garlic elixor works equally well for this purpose. The garlic elixor or garlic extract consists of concentraited garlic juice that comes in a small bottle with an eye dropper. It is potent enough that if you add one drop to a cup of water you can smell the garlic clear across the room.
To dose your frozen Mysis with the garlic, you just follow the frozen Mysis as usual, placed the thawed Mysis in a small cup with a little saltwater from the tank, and add 1-2 drops of the garlic concentrate to the cup. Allow the thawed Mysis to soak up the garlic for about 10 minutes and then target feed the Mysis to your seahorses as usual, using your baster or whatever implements you normally use for target feeding. (In your case, the same technique can be used to infuse your live foods with a trace of garlic, PonyGirl.)
However, I don’t believe you will find the garlic elixor at your local fish stores or aquarium shops; rather, I think you’re more likely to find it at your local grocery store or supermarket.
Finally, I am told that Vitamin B12 is also worth considering as an appetite stimulant. You’ll need to obtain Vitamin B12 tablets intended for human consumption from a drugstore, health food store, or supermarket, crush one to a fine powder, and add the powder to a high-flow area of the aquarium about one hour before the scheduled feeding to stimulate appetite.
If the affected pipefish is showing signs of respiratory distress such as labored breathing or rapid respirations, PonyGirl, then you should also consider the possibility of gill parasites, which could be addressed through the use of metronidazole or possibly a freshwater dip (if you feel the pipefish is strong enough to tolerate the stress of being captured and subjected to the freshwater dipping procedure.)
Let me know if you would like to try the metronidazole or perhaps a freshwater dip, PonyGirl, and I will be happy to provide you with instructions for performing the freshwater dip safely and/or the best methods for administering the metronidazole.
Pete Giwojna, Ocean Rider Tech Support