New tank/Fish dying

Question
I have setup a new 46 gal saltwater tank with about 15 lbs of live rock. It has been running for over a month and I added 4 damsels. The 2 larger ones died (good for about a week and then went on side and died)

I waited another week and got some hermit crabs crabs to help with brown algae. Again, the larger crabs seem to be dying and I tested the water and all looks good.

Am I doing something wrong?

Answer
Hi Bill,
Sorry to hear about your troubles and losses. How deep is your substrate? 15lbs of live rock is pretty low, the system will take longer to cycle without more live rock for the bacterial filter to develop in. The rule of thumb is 1 pound of each substrate and live rock to each gallon of water volume to maintain a healthy system. Now it could be any number of things doing in your pets. If you are getting good tests of your water(Ammonia, Nitrite, and Nitrate are all Zero?) it could be your just getting poor specimens, already sickly or damaged through collection or housing at the local fish store(LFS). This unfortunately is quite common. So test your water quality again, make sure you have good water movement and filtration, I personally would add about 30 more pounds of quality live rock, look at the tanks location and make sure that its not getting too much sunlight or anything around that can pollute water quality(some common house plants can drip or lose leaves that are toxic). Look at feedings, hermits will need more than just algae, especially if the larger species, a new tank less than a year old with only 15 pounds of live rock wont provide enough food for them. The same goes for the damsels(not the algae part) they will need to be fed at least 4 small feedings a day.
Dont feel like youre doing anything wrong, marine aquariums take a bit more time and patience is all. Get yourself some more live rock, test your water quality(I'll say this repeatedly to everyone) every 2 or 3 days with a new system, and hang in there. You'll get it. And feel free to ask as many questions as you need to ask, thats what we're here for.