Lets get to the basics, forget the calcium / TDS all of the non essentials at the moment. You cant keep a fish alive, why are you worried about calcium?
1. How much live rock do you have running in your system?
2. You say you have powerheads, is there any surface water agitation? Try to make the surface of your water as rough as possible, this will provide oxygen to your tank, fish cant live without oxygen.
3. Do you replace evaporated water with new, FRESH water? Im guessing you should have at least a 1/4th of a gallon a day evaporation. If you dont top off, your salinity will creep to unhealthy levels.
4. I dont think your lighting is strong enough at all to support coraline algae, much less zooanthids.
5. When you have ammonia, a 5 gallon water change isnt going to do much at all, If you have any readings of ammonia I would at least try a 20 gal water change, just my opinion.
6. Are you running any type of sand? You need some type of surface area, sand or live rock for bacteria to live on.
7. Torn fins are a sign of ich also, have you seen any spots on your fish before they die?
8. Test kits last tops 1 year, then they become innacurate. Chances are that if you bought your API test kits from a commercial pet store, they are already a year old. Look at the lot numbers on the bottles. I checked my local pet store and found all of their test kits to be over a year old already. I bought my new set online and they were made within the last month.
9. Get back to the basics, Rock / Flow / Light. Read up on alot of the newbie manuals and reefkeeping.com tutorials. When in doubt, Waterkeeper will always help out. Hes a great source of knowledge and will always help a new reefer out. READ as much as possible.
10. Most swing arm hydrometers are temperature sensitive. Most are calibrated to test water at 77 degrees, If you take water to a fish store to test and they use swing arms, remember temp plays a huge part.
11. What is your temp running at? How are you heating your tank? Are there large swings in temperature from night to day? Larger than + or - 3 degrees?
12. Resist temptation, when your tank is young only introduce one fish at a time.
13. We have all learned this hobby the hard way, we all have lost fish, we all have had a near mental breakdown over our tanks at one point in the hobby or another. Keep your head up high, dont get discouraged and sometimes its best to just walk away from the tank and let nature take its course if you know everythings in check from the ability that you have to create. Stress kills fish, Huge monster hands in a tank moving their whole world around, that would stress me out lol. Keep an observant eye at your ecosystem and try to remember your taking an animal from the sea to your livingroom. Try to re-create the best environment you can for him/her. Your doing a great job already on asking questions and researching. Keep up the good work!
