Why do my fish keep biting the dust?

Question
Hello Chris-
I have a 55 gallon tank with 4 gouramis, one kingtail  catfish, 4 platys, 3 mollys, 3 rosy barbs, 1 rainbow shark,1 leopard-something (i forget--sorry!)2 silver dollars and two filters (1 penguin and 1 bio-wheel--both very large).  My pH is 6.8, ammonia is 0 and nitrite is .5--I do partial water changes every two weeks and i would estimate about 10 gallons(??) of water per change (exclusing when the tank is siphoned).  I have gone to PetCo and purchased a majority of my fish--and although my level are all normal (minus the hardness-however I live in DC and have asked PetCo if i should be doing anything for the hardness levels and they explained that it is just typical of DC water and is nothing to worry about)--thus far, i have had about 8-9 fish die on me! Two were carrying fungus when i bought them and i took them back to PetCo and saw the entire tank was contaminated--but most of the other fish have absolutely NO symptoms and just croak!! My two clown loaches just died, and i have had various other gouramis, mollys and platys die.  Just today i noticed my two other gouramis have fin rot!! It also looks like the silver dollars might be coming down with fin rot.  WHAT am i doing wrong? I absolutely cannot figure it out! All my levels are right on the money--perhaps i should just stop buying my fish at PetCo? Please help!

-One other question--in my tank I have a sand/gravel mix (sand came from a healthy well established tank) and a lot of the sand which lines the bottom of the tank is turning black! Is this normal? Perhaps from lack of air or just bacteria build-up?  

Thanks for your help and advice!

Answer
Hi Corinne;

If it is a newly established tank I would say the deaths are from toxins that build up during the Break-In Period. They spike for a while and then go down as the tank stabilizes. It can indeed cause the death of fish in the first few weeks. If it is new, here is a link to my page on new tanks to help you know more about what is going on;

http://www.xanga.com/Expert_Fish_Help

Find a little owner-operated store run by people who have real experience. I know there are a few employees of big chain stores that are good but, by and large, there are far too many sick fish in those places. The employees are generally very limited on knowledge as well as limited on allowed resources to care for the fish. As a result, the fish are often not cared for properly and not diagnosed and treated when they are sick. Sad but true.

If there is crud in the bottom of the tank it means the gravel needs to be vacuumed. All tanks need a 25% water change and gravel vacuuming at the same time once a week, every week. If a dirty area in the gravel gets disturbed and poisonous gasses come up from it, it can kill nearby fish in just seconds. That's why it's important to vacuum weekly and also avoid overfeeding.

What looks like finrot could actually be nipping by the other fish. I don't know what a "kingtail cat" is but the shark or the "leopard" could be causing trouble for some of your other guys in there. Rainbow sharks can be very nippy guys. If you could take photos of the two mystery fish, upload the images to a free image hosting site such as photobucket.com, screenshots.cc, or imageshack.us, etc. and send me the links I could take a look and see if they could be aggressive fish.

At Your Service;
Chris Robbins