salt water tank fish start to die after a month!

Question
hey..i have probably a few worms in my tank..is that normal and are they dangerous? will the meds kill them? its those spiky lookin worms..im concerned about the worms being in the tank with the fish..
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theres no rust or anything..just brown algae growing on the sand and stuff..i have been medicating the tank for a good two weeks now..how long do you recommend i do this procedure? and should i put a tester fish before i do a water change? or should i do the big 50% water change and run the skimmer for two weeks THEN get a test fish?
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i have taken my water in to a local fish store and had it tested for ammonia and all that..and everything seems fine..so they say..pH? i thought that salt water tanks didnt have to worry about pH levels? my fish were doing fine until one day it just all died..i have a jebo skimmer but i have turned it off for now because i am medicating the tank for  parasite and worms and all that JUST IN CASE that IS the problem..i also have an oddysea canister filter and it works fine..right now it doesnt have the carbon because of the medication im using..the salinity right now is at about .021..water is crystal clear and nothing noticable..i USED to have worms but i think the medicine killed them already..please help!!
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i just recently finished starting up a semi-new 30g salt water tank..my friend had a 18g and i used his water and sand and added the rest..my tank cycled really fast considering my friends 18 gallons of developed salt water/sand..so i did all of my cycling and water checks and everything is fine..i started off with a damsel and it was great..added two clowns and the damsel died..but the two clowns, i think beat him up..i also added a little blue leg hermit crab and a goby..that was fine..after a good 2 weeks of good behavior..one of the clowns died..and i didnt know why..checked my water and it was good..temp was at 80..no nitrite no nothing..so i thought it was nothign..i then a few days later added a foxface and that looked fine..then the very next day..i find that my other clown AND the foxface are dead..why? i dont know!!..so then my goby dies not too long after and then the crab..and every fish i add in now die within minutes of getting in the tank..i do the proper procedures of floating the fish..my friend told me it might be electrical..so i switched out the heaters and same thing happens..except they actually live probably a few minutes longer..but same results..can it be parasites this soon? and can they kill that fast? ive actually been putting in some medication for the parasites if i do have some..but fish are still dying..please help me so i can keep my tank and watch some colorful fish again :(
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CHECK SALINITY, PH, AND AMMONIA. THE ONLY THINGS THAT I KNOW WILL KILL FISH THAT FAST. TAKE A WATER SAMPLE, ABOUT A QUART INTO YOUR LOCAL FISH SUPPLIER AND LET THEM TEST IT FOR YOU. WHAT TYPE OF FILTRATION DO YOU HAVE? DO YOU HAVE A PROTEIN SKIMMER? SEND ME MORE INFO. GERRY
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If I were you, I would continue with the meds as long as you feel is necessary to get rid of the parasites you may or may not have. Wait a couple of weeks, do a major water change- at least 50% and change all the filter media. Let the tank run with the skimmer for another two weeks. If you rush it you will have very limited luck. It's hard to wait while you sit there with an empty tank but it will pay off in the long run. Check, also to see if any metal fell into the tank or is rusting in or around your connections. Gerry.
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STOP THE MEDS. CHANGE ALL THE FILTER MATERIAL. ADD THE CARBON BACK IN. DO THE WATER CHANGE. LET IT RUN FOR A FEW DAYS. BUY TWO DAMSELS - GOOD TEST FISH - CHEAP AND HARDY. YOU COULD EVEN NAME THEM "CHEAP AND HARDY". ANYWAY, SEE HOW THEY DO AND GET BACK TO ME. THANKS, GERRY.

Answer
they can become a problem if allowed to multiply. There are ways to control them. Go to about.com, click on hobbies and games, click on saltwater aquariums and you will be at a site where you will find many answers to many things, including bristleworms.