Salt Water Tank/Fish issues

Question
Thank you for your quick reply.  I have only changed the water about once a month and about 15 gal at a time for a 55 gal tank. (Probably a big part of the problem.  I don't have a protein skimmer but am going to read up on them now.  My Angel lasted about a month, the Yellow Tang for about 5 months, the wrasse about 5 months.  I did a 15 gal water change yesterday and today I did the chemical test:  PH = 8.0/amonia=0ppm/ nitrite = 0ppm/nitrate = 0ppm/KH=13.
I went onto the site and read up on the Aptasia.  I will defiantly be getting rid of these.  We were so excited and thought we got a bonus with our live rock having these anemones, I guess I was wrong!    I will also look into an air stone for the next water change.  You have already been a great help and appreciate any pointers you have to offer!
PS  Sorry I put this whole paragraph in the Rating section by accident.
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The text above is a follow-up to ...

-----Question-----
Hi, we are having problems with our tank.  We have had it up and running since the end of Beginning of May.  We have lost several fish not from ich.  We have no clue why we are loosing them, they look great and then the next day one is dead.  We have lost a Tang, Angel, couple Damsels,Warrase. We have lost many of the snails.  All of the chemicals come out fine.  I have noticed alot of algae (?) red (burgundy) looking stuff coating everything.  We have a crushed shells on the bottom and live rock and one anemones(sp).  We have many blue crabs and lots of these brown looking anemones that have appeared on the live rock.  Our tank seems to be getting dirty pretty quick and we have an ehime(sp)which we were told would take good care of the 55 gal. tank.  We only have three fish left.  a clown and two damsels.  I don't know what we are doing wrong.  No signs and we are being extra careful not to over feed.  Any help you could offer would be greatly appreciated.  Frustrated in Wisconsin
-----Answer-----
Hi Jodi,
   I'm so sorry to hear what you are going through, it is so frustrating, I have been there. There is something wrong in the water, almost guaranteed. How often do you do water changes? What quantity do you change? Do you have a protien skimmer? How long were your fish alive? What is your nitrate reading? It takes a lot to kill a damsel. Tangs and angels are far more delicate. The filter system despite what you were told is either not handling the bioload or the medium in the filtration system may be clogged and needs to be cleaned or replaced. Buy some amquel plus, by Kordon. Good for nitrites, ammonia, nitrates and conditioning water. I would do a 20% water change, right away. Please recheck your levels, ammonia, nitrites, nitrates, ph, and salinity. Nitrates are the food the dark red algae feed on. The brown looking anenomes are aptasia, aka rock anenomes. You should get rid of them asap. Go to about.com, click on hobbies and games, then lower left click on saltwater aquariums. You will be at the site run by stan and debbie hauter. Ther you will find info on how to get rid of aptasia, as well as almost any other saltwater subject you can think of. Please get back with some answers and I will do my best to help you. Good luck, gerry.
P.S. When you get water ready for changes do it 24 hours in advance with a powerhead or airstone in the bucket along with a heater to keep temp same as tank water. By doing this curing process the new water you introduce to the system will be more readily accepted by the animals and bacteria colonies needed for nitrification.

Answer
Hi Jodi,
   Your readings are right on the money. Once a month should be ok on the water changes at 20% unless your bioload has increased for some reason. Filters clean and performing? Could something have dropped in the tank, like a coin or something? If your fish were eating and acting fine for five months and all of a sudden they up and died, there has to be a reason. All diseases do not manifest visual signs but they do affect the fishes behaviour and eating habits. Have you used any medications or other additives? Would love to have the answer somehow present itself through close scrutiny. Get back to me, gerry....