Hello fellow aquaponists:

Recently, I have experimented with a new system (i.e. grow towers above a single fish tank that houses the fish and water pumps, see <http://www.aquaponiclynx.com/WordPress/wp-content/uploads/2011/03/t100-1-Medium.jpg>) and have experienced totally mortality of all my tilapia despite maintaining key environmental factors (i.e. pH ~6.8, regular removal of solid waste, ammonia 0ppm, nitrate 80ppm, nitrite 0ppm, temperature ~22-24 °C, new system was situated in heated indoor environment). Below is the timeline of events:

1. In the new system, I cycled the water and there were no nitrites and low levels of ammonia; as well, nitrate levels were increase suggesting that the key nitrifying bacteria had become established in the bio-filter.
2. I transferred 11 fish (i.e. ~5” in length) from an old system to the new system.
3. A couple days afterwards, key environmental factors were still being maintained but the tilapia had not been eating any of the fish pellets they were fed. The tilapia continued this behaviour until their death.
4. Two months later, I discovered one of my tilapia had died. Then subsequently over a course of fifteen days 10 more fish died. Within this time, we tried to improve the situation by cleaning the tank, changing the water in the system, and reducing feeding frequency and the amount of feed. Upon inspection of the external surface of bodies there were no signs of physical trauma or pathogenic infections.

I have several possibilities, which are not necessary mutually exclusive and there is not a clear cause and effect relationship, as to why my tilapia may have died:

Psychological shock: When the tilapia were transferred to a new system, which may have caused the tilapia to lose their appetites. As a result, they starved themselves and made them more susceptible to pathogenic infections.
Environment shock: Environmental factors, other than the aforementioned water conditions which are at acceptable levels, may have caused stress, e.g. psychological shock, to the tilapia in the new tank.
Nitrate shock: Moving the tilapia from an old system with >160ppm nitrates to a new system with much lower nitrates may have stressed the tilapia. As a result, they starved themselves and made them more susceptible to bacterial and pathogenic infections.
Electrical shock: Electrical leak from electrical components in the fish tank may have “shocked” the fish so much that they became psychological and/or physiological stressed.

I know this system can work because others have purported it to be the case. If anyone has any insights on this issue and could help me, I would be very grateful. Especially any ideas on why the fish would not eat anything.