OK For one you learned this ....... It is very hard to trust a LFS , they sell fish for a living with most of the time no regard to the fishes health .. i was told i could have a moorish idol in a 20 G Tank ............. However i knew this wasnt true i was just testing the store ................ anyway what your going to want to do and i know this sucks , but it has to be done ........
1. Take all of your fish back to the LFS for credit if they will do that , if not there are other LFS that will accept fish for credit. If you dont do this the most likely scenerio is your going to lose your fish and be left with nothing to show except a few damsels ...... and honestly who wants to be stuck with a damsel
2. IF you have LR and LS, GREAT! if not you need to get some , it is expensive but is really a must as it produces natural filteration. A general rule is 1.5 Lbs per gallon , but i have 40 lbs in a 55 and my setup is fine. Also mix LS and Aragonite sand and use that is you flooring
3. You can keep the water you have or you can just start fresh which i belive is the best way to go.
4. If you go fresh great your on the right track . fill the tank with Pre Mixed (24 hours) Salt Water with a power head running in it. Put the water in the tank.
5. Drop a Cocktail shrimp or 2 in the water , this will get your cycle going for you ,
first your going to see your ammonia spike pretty high , this is great it should take about 3-5 days for it to spike , after about a week or less with ammonia your ammonia will turn to Nitrites , then slowly your nitrites will go down and turn to nitrates when your Ammonia hits 0 and your nitrites hit 0 your nitrates should be at about 40 wait a week for them to slowly go down ........ THEN do a 25% water change with premixed water. wait a couple of days and add your first fish , your first fish should be a hardy fish , BUT i would not go with a damsel , A False Perc Clown will do good they are pretty hardy. see how he does for about 2 weeks and go from there
That is the proper way to cycle and i hope it helps
Depending on how big your tang was i would say it probably shouldnt have been in there because either way he was going to outgrow the tank ..... people say tangs need a 90 gallon set up ..... I dont find this true JMO though .... i have my yellow tang in my 55 with a maroon clown and they are both very healthy and doing awesome. He has Adequate swimming room and lots of rock work to feed off of.
The thing with this hobby is Patience , without it your doomed there is no way around it.
Not your fault you were misinformed by your LFS but in all fairness Impulse buying in MArine Aquariums is a disaster ..... alot of research needs to be done by the potential owner because you have to be able to satisfy a fishes needs that it has in the wild like with your tang it needed to have algea sheets to nibble on during the day , thats something your LFS probably didnt tell you . Also with tangs they arent very hardy you were misinformed there also . They need an well established aquarium with optimal salinity of 1.024-5 , they do not like anything lower than that , they will adapt but it will hrt them in the long run .........
A Last key Point is . Dont let this frusterate you , and scare you away from such a great hobby ..... you can have success with saltwater , everybody has made a mistake and everybody has learned from it . get yourself a good saltwater book and study up , the best one i know of is "The Conscientous Marine Aquarist" by Robert Fenner
Anyway
I hope this helps you out
and good luck in the future
also sorry about your loss
