attacking fish

Question
hi I am wondering if you can help me I have a Rio 180 fish tank which contains 4 dwarf guaramis,14 medium neons,2 female fighters,6 male guppies,2 angels and 2 silver sharks. My pH is 7.0 with no nitrate and 0.3 ammonia my tank has been running for 6 months and gets a water change every week taking out 6 gallon on each water change.My problem is I have been loosing fish and I do not know what fish is attacking the other fish. I was wondering if you could tell me out of all the fish I have what ones could be doing the damage hope to hear from you soon thanks.

Answer
Hi Susan
What's happening to the fish that died?  Any symptoms before they died?  And what types of fish are dying?

If it is indeed a fish that's killing them, my guess would be the angels or the sharks, possibly even the gouramis.  Hard to say at this point.

It may also be a water quality issue you're dealing with.  Something's wrong with your tank....it's been set up for 6 months you said.  So, your tank should be completed cycled, and the ammonia and nitrites should be reading 0 ppm, and you should be showing some nitrates present, those though you want to keep under 20 ppm.  With you saying there's no nitrates present, and some ammonia present, that tells me the tank hasn't cycled, or something has killed off the beneficial bacteria causing it to cycle again.  In case you're not familiar with the cycle process, here's a link, it's the first article on the page:

http://freshaquarium.about.com/od/startupcycle/Step_5_The_Cycle.htm

You said you do weekly water changes of about 6 gallons, I'm not good on conversion rates :)  but you want to change out about 25% of the water weekly, along with a good weekly gravel vacuum.  You should probably change out a few more gallons each week.  But that most likely isn't what's causing the lack of nitrates present.  How often do you change out the filter pad in the filter?  Have you recently used any medications in the tank-if so what kinds and for what?

Christy