So going on little information as far as actual readings in your tank, and assuming that the fish were not diseased or had no parasites, since you said that they were stuck to your power head or filter intake, I again will ASSUME that they were too weak to get away from it. With that being said, do you have any surface agitation? Surface agitation promotes gas exchange. In freshwater you have your bubblers I assume. In saltwater that's not needed however you do need movement of water at the top to some degree. Without salinity level readings, pH levels, ammonia, nitrite, nitrate, it's really hard to say at this stage.
Being new to the hobby, I realize this is a bit discouraging as we all await the day our tank is cycled to get fish. Here are a few things that in my opinion may help you (others may agree or disagree):
1. I would look into quarantining all fish before they go into your main tank. One infected fish added to a healthy tank can wipe them all out. I would do this for 2-3 weeks...yes, that long because once you get marine ich, it's in the main tank until you take all fish out and provide it no host! Never use meds in your tank. It can kill inverts.
2. Always test your water before adding. I would test myself and then take a sample to your LFS
for them to test. Some may say this is overdoing it but when you spend $70 on a fish, I feel it's a good way to ensure you are on the right track starting out.
3. Make sure you acclimate anything you put into your tank properly and slowly. Most fish stores keep salinity at .020-.022 . I'm going to say this and I know I will catch grief from someone on here but this is NOT a good level to keep your tank at. If you will research what actual salinity levels are in the ocean, you will find that many range from a .025-.027 (the red sea has a higher salinity level). Your fish will do better with higher levels in the long run.
4. Watch your fish eat before you buy them. Look for any white spots on them, lesions, swim patterns, etc. It's always best to research them then buy. We all have gone in for one thing and come back with things that looked awesome but we didn't know diddle about. Some fish are not comparable with others.
Honestly, each of us have what works for us best. If you will take your water to your LFS
and have them test it that for everything, tell them what happened, that may get you some answers. Sorry for the long dissertation!!