fish dying again

Question
We have had a saltwater aquarium since Feb. We started slowly adding fish, coral, plants, rock. All the fish were dead by Easter weekend.(except clownfish,crabs)
We started again adding fish one at a time. We had a tang, sailfin, shrimp, damsel, lawnmower blemy and a goby, lots of crabs.....plant grew back. I have been doing 20% water changes every two weeks..(we never did water changes the first time around, we were still learning) ANyways, yesterday our one plant let off all of this haze and I did awater change thinking that everything would be okey. This morning  the tang, sailfin, shrimps, damsel are dead. Oh yah, we had a clownfish all of this time since the beginning, and he is always alive....but sits floating at the top of the tank.
I have noticed that both times this has happened, it has been around six weeks and they start dying, also, when the fish are dead, it has been during the night, and also, in the morning all of these tiny worms, shrimp, bugs are all over the glass and all over the place when usually you don't see any of these.
Our tank is 60 gallon
After testing Nitrates, Nitrites, PH, Ammonia, Iron,etc...the only thing is our wateris extremely Hard.


Answer
what kind of water flow do you have in the tank? if the clown is sitting at the top it could mean there is a lack of oxygen in your tank. get a couple of good powerheads and point them so they are disturbing the waters surface, this helps keep your PH stable and aids gas transfer into your water.

the plant you had, do you know the name of it? what it probably did was spawn, it releases spores into the water to re-produce.

how much liverock do you have in your tank? in addition to having the powerheads disturbing the surface, get some to pump water in and around the liverock. this will take water to your anerobic bacteria and they will keep your water pretty good, you will still need water changes to keep nitrates down.

the best advice i can give really is to keep the fish you have for now, give it a good month or so, keep up with the water changes and water testing. also your tank is way too small for tangs. could also be the root of the problems is overstocking. your aiming at 1" of fish per 6 gallons of water, remember to calculate this taking into account the fish full grown size.