Backyard Aquaponics
A place to discuss aquaponics

So this is our second time ordering tilapia fingerlings and...neither has worked out very well. We've had this batch since March 9th. They were doing quite well at first, and we went over a week with no mortalities. Our levels were all perfect when we first received them, and we monitored them very carefully. We started with 125 fish, and so far have lost about 46 of them. The concern is that the last time we got babies, we ordered closer to 500 and only 1 survived (he's still alive and kickin, though).

Our levels currently:

pH: 6.8 (a little lower than ideal, but we've been nervous to treat it in case it stresses them out)
Ammonia: <0.25ppm
Nitrite: 0ppm
Nitrate: 80ppm

The fish themselves appear healthy and are eating twice a day with much enthusiasm. We have okra growing over the tank that are also doing quite well. The fish have been growing and seem completely fine with no abnormal behaviors, right up until we find them dead.

Water temp?

6.8 is fine for Tilapia. Do you have a UV sterilizer hooked into the system?

With apparently no signs of disease I would suspect either that they had a problem with some physical parameter - spike in ammonia or nitrites, problems with lack of oxygen, problems with drastic swings in pH, some toxin getting into the water, OR that they have some internal infection. A UV Filter hooked to the inflow into the tank might help with this last one and prevent other fish from being infected. It doesn't need to be permanent, you just need to get control of the situation. Based on your previous experience if it were me, I'd probably get the UV Filter.

what you basically need to do is find some way of buffering your water. We use mollusk shells (without the mollusks) in our biofilter. the nitrosomas and nitrobacter actually live on their shells and the shells are gradually dissolved into the water providing a carbonate buffer to neutralize your pH.
Stocking wise 60Kg/L/hr is an upper limit and 3% biomass/day as a feed guide.
Zero Nitrite is good and the nitrate levels will be absorbed by the shells.
The fish exhale ammonia and the bacteria convert it to nitrate. We use a tower type biofilter filled with layers of shell and a natural updraft of fresh air. It is essentially a biofilter that acts as a gas scrubber (NH3 scrubbed out from the top and O2 scrubbed in from the bottom).

best of luck

Sounds like you have something unique used as a bio-filter. I'm very much interested to see what you have and would like to see some diagrams.

My main interests are along the same lines, such as using a Mechanical filters to remove solids from the waste water.

MT, first done anerobic, second done aerobic before being used.

On the subject of fixing ph, reducing acidity, why would adding magnesium or calcium to the water, or something alike, not work? Am I being stupid? I'm a newbie.

There really isn't a pH problem here CP so there's no need to adjust the pH but FWIW the form that you add the Calcium or Magnesium has a lot to do with the effect it has on pH. Normally you'd add something like Calcium Hydroxide or Calcium Carbonate to buffer the pH up. Carbonates are part of a buffer system and hydroxide is a strong base so most of the change in pH comes from these. I usually don't think of Calcium or Magnesium being used to change the pH.

wrt your dead fish did you notice how they looked ie were their mouths open or not, widely gaping or just open ? this sort of info give people like me some idea as to the nature of the deaths. eg some years ago after a rain storm there were fish deaths in a local pond. on inspection the kill was universal in both species and size. this is significant because low o2 levels kill biggest first. turtles were out of the water but alive, again significant. approach by humans will send them scurrying. so the turtles were injured in some way. without going into great detail the fish had their gills burnt by sulphuric acid from runoff from untreated acid sulphate soils. the soils should have been treated (buffered) with lime to neutralize the acid.
Having said all that I am then drawn to CP's comment that okra was grown above the tank. field hands that work with okra have to wear protective clothing ???????
okra is grown for its spicyness and most spices are an acid of some sort eg (chilli capsican) an acid, clove oil also an acid is a potent anesthetic to all animals inc fish and crustacea
I would imagine that it would be a bit like companion planting only placing plants with livestock.

wrt to mechanical filters look at some of the mining engineering sites eg coal in regard to hydrocyclones. these machines have no moving parts, can be home made with the wastes exiting the bottom as a slurry and the clarified water out the top to be dropped into the scrubber.
food for thought.