Fish Dying in Cycled Tank

Question
QUESTION: Help, I am desperate! I have had my fish tanks for going on 3 years now (ranging from 10 gal Betta tank to 60 gal cichlid tank) and during the past two weeks I have lost 2 tiger barbs and a cherry barb in my cycled 40 gal tank.
My water tests are showing a reading of .25 for ammonia, 0 for nitrate and 0 for nitrite. I do weekly partial water changes with treated cured water of approximately 12% (I change 5 gals in a 40 gal tank, not counting top off water that is added as needed),  My tank temp is at 80 degrees, and it is a planted tank.  I have added a plant to my tank, but that was after I had already lost a female Tiger Barb and a male Cherry Barb (both 3 yrs old, as they were my first fish). I have not added any new fish to my tank in over 3 weeks (added one small panda corydora 3 weeks ago who is doing fine and schooling with my other 3 pandas)  
My 40gal tank contains the following fish: 3 Tiger barbs and 3 Albino tiger barbs, 1 kissing gourami, 1 black sailfin mollie (he lives here because he killed too many of my male mollies in the mollie tank), 3 Gold batbs, 4 panda corydoras, 3 cherry barbs and 1 clown loach. I had an adult chinese algae eater until 4 weeks ago when he jumped out of the tank and my Boxer made him a snack before I could save him from her.  
Today, after finding the male Tiger Barb dead, I noticed that my clown loach was hanging out near the filter, so I checked the water and got the readings above.  
The fish that died showed no signs of illness (I have been very fortunate to never had an disease in my tanks that I am aware of, but then I am very careful with my fish, as I know how delicate they can be).  This is distressing to me.  If it had been just one Tiger Barb, I wouldnt have thought much about it due to probable age being a factor.  But now the mortality rate is climbing and I am getting scared.
I am doing a partial water change this afternoon after the water has been sitting for 24 hours, but wonder if this will be enough.  I can not explain why I have a reading at all for ammonia, especially since my nitrates and nitrites are at 0, and I have two filters (a 40gal filter and a 35 gal filter- both Whisper hanging power filters) that have been in the tank for over a year.  I change filter media monthly, and rotate between changing the media, doing one filter a week a few days after the partial water change, so I do not upset the beneficial bacteria growth in the tank.  Am I just over reacting? Or am I doing something to kill my fish?  I am really attached to them all (everyone even has a name to match their little personalities, and I spend time with each tank daily to look for anything out of the ordinary with my fish)  Also, the barbs have been hanging out on one side of the tank, while the gourami and mollie (who are inseparable), pandas and loach have been swimming freely. Everyone is eating fine, they get a diet of Spectrum Flakes and pellets, peas on a weekly basis (which they all LOVE), frozen bloodworms, and  brine shrimp a few times a week. Ive even given them blanched zucchini or oranges on occasion for a treat (my pleco adores this so when I give it to her, I share some with the other tanks and remove whatever isnt eaten before I go to bed).  Any assistance you can offer would greatly be appreciated!

ANSWER: Hi Marcie,
 You sound like a very conscientious fish keeper and I cannot see anything that you are doing wrong.  You might increase the water change from 12% to 25% if you can.

 Most likely the deaths are due to attacks by other fish, rather than water chemistry.  Keep an eye on things.

-- Ron C.
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>


---------- FOLLOW-UP ----------

QUESTION: Thanks Ron!  Seems that things are back to normal in the tank, and no more deaths (except for a new Tiger Barb that jumped out of the tank in the middle of the night).  I have increased the amount of the water change to 20% the same day i wrote you to bring the ammonia level down.  i think it may have been a dead grass plant that was hiding behind a rock which I located when cleaning the tank.

I have added 3 new small Tiger Barbs, as I was concerned that my school had gotten too small and the remaining Tiger Barbs would start harassing the other fish in the tank.  Now I have a school of 8, so they should keep any nipping between themselves.

I have one last quick question regarding my clown loach.  She seems to be having a great time hanging out on the filter grate that attaches to the water uptake hose.  As dead fish are usually sucked to the grated portion I became alarmed when I saw her there!  At first, i thought the worst, that my loach was dead.  But I noticed that her colors were fine and as I approached the tank, she seemed to "wake up" and swim away, only to go do the same thing on the other filter!  She has scared the heck out of me before by "napping" in the middle of the tank on the gravel, and I found out that this behavior is typical of clown loaches   I still get a bit caught off guard when I see her napping on the gravel, then remind myself she likes this, its typical of her species and that shes fine.  So does this grate thing sound like another one of their antics?  Shes eating fine, dancing for her food and attention, but seems to enjoy the filter sucking on her.  Just scares the heck out of me.

Answer
Hi Marcie,
 I have never kept clown loaches but the behavior you describe, namely "napping" is exactly what I am told that they do. They look really dead, and then get up and are fine.  It is a very peculiar thing, but perfectly normal for clown loaches.  

-- Ron C.
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>