Unknown Cause Of Fish Deaths

[Boston Jack]

I have had a 44 gallon fresh water tank for at least 10 years. I recently switched the fish to a 44 gallon hex to fit into the corner of dining room when putting new kitchen 3 months ago. I used the Biowheel Penquin filter and rock substrate from original tank and transferred about 15 gallons of the originial water.
About 6 weeks later I lost a 8 year old grey tetra that was prone to ragged fins and sickness about once a year. I was not that concerned based on the prior bouts of sickness and it's age. However about 3 weeks later the 6 year old other grey tetra that never had been sick died (based this on loss of mate).
The water quality is fine and shows no nitrates or ammonia and the Ph is 7.8. Today I found a 6 year old Bala shark dead at the bottom of the tank. Last night the Bala fish was taking turns playfully chasing each other as they have done for years after eating. I saw no redness on the body of the recently departed, the gills and fins looked fine, nothing that showed external disease. I had introdueced two gourami after the tetras died 3 weeks ago however I do not figure this into the deaths in the last few months. Prior to these deaths I have had none in the last 4 years and the ones I lost were to disease.
I am sitting here scrathing my head as I know it is not new tank syndrome because I replaced the Biowheel within one hour from one tank to the other. Can anyone please provide some suggestions if they have experienced something similar in nature

 

[karin]

Did you test for nitrites? You did not say. I have had the nitrite metabolizing bacteria crash before and the ammonia metabolizing bacteria were fine. That is I had a nitrite spike not an ammonia spike and for some reason my experience, limited granted, leads me to believe that nitrite metabolizing bacteria are weaker or more fragile. Just an idea.

 

[Boston Jack]

Thanks for your reply Karin. I actually did test for nitrites but not for nitrates (mispsell). So indeed I need to pick up a test kit for nitrates and possibly a plant or two. I have been negligent of water changes in the past because of good results of tested water quality. I have been a stickler on changing media and carbon on a timely basis. So I would like to rule out some variables in the future by nitrate test and more water changes