could something kill the nitrifying bacteria and stop the cycle?

[Roe Sie]

If anyone has any ideas, I'm stumped. My system is 6 months old, and had cycled just fine. I added 35 tliapia fingerlings. I lost 6 initially, then they were doing great for 3 or 4 months while I fine tuned PH and temp (PH is now 7.2 and temp ranges from 79-84).

For the last month or so, I've had ammonia spiking really high (using am-quel to try to save the fish) and absolute zero nitrite and nitrate readings. I'm losing a tilapia about every three days now. 

[Vlad Jovanovic]

Ah, yes...I see. "breeze" is a synonym for "fly ash". In that case it is in the realm of possibilities that they may be leaching small amounts of toxic metals. Don't take this to mean that that is definitely whats going on, just that it is possible like you alluded to.

http://en.wikipedia.org/wiki/Fly_ash

This from a study on the leaching problems of breeze or fly ash...

...."Use of lignite in power generation has led to increasing environmental problems associated not only with gaseous emissions but also with the disposal of ash residues. In particular, use of low quality coal with high ash content results in huge quantities of fly ash to be disposed of. The main problem related to fly ash disposal is the heavy metal content of the residue. In this regard, experimental results of numerous studies indicate that toxic trace metals may leach when fly ash contacts water. In this study, fly ash samples obtained from thermal power plants, namely Soma and Tunçbilek, located at the west part of Turkey, were subjected to toxicity tests such as European Committee for standardization (CEN) and toxicity characteristic leaching (TCLP) procedures of the U.S. Environmental Protection Agency (U.S. EPA). The geochemical composition of the tested ash samples from the power plant show variations depending on the coal burned in the plants. Furthermore, the CEN and TCLP extraction results showed variations such that the ash samples were classified as `toxic waste' based on TCLP result whereas they were classified as `non-toxic' wastes based on CEN results, indicating test results are pH dependent". 

Anyways...it would probably be a good idea to get the concrete blocks out of there on the pH related basis alone.

Don't know where you read that concrete in contact with AP water was OK, but there are a good number of folks on this forum who have found that it's not. And ended up having to drain  the tank, and coat the concrete with a fish/bacteria safe coating, or removed the concrete blocks inside their water to solve their high pH issues...

[Roe Sie]

I'll be getting them out of there and I'll let you know how it goes. thanks for all the great advice.