Aquaponic Gardening

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I have a 180 gallon tank. I added 50 Tilapia fingerlings in mid-August.  They have been growing very well since then, and have appeared very healthy.  About a week ago, I started finding fish around the tank that had jumped out and flopped around until they died.  

Around the same time the fish started jumping out, they stopped eating.  I had them on an automatic feeder and noticed food accumulating on the water surface.  I have now stopped feeding the fish until I diagnose the problem.  

Currently pH is 6.8, Ammonia and Nitrate are near zero.  Water temperature is 67.4F.   The water smells fine and the plants are healthy.

My initial theory was that the water chemistry was off, but I can't find any evidence of that.  Fish in the tank appear healthy other than not eating.  

I live in Virginia, and temperatures have cooled quite a bit in the last couple of weeks, with typical water temp falling about 10 degrees.

I have two theories right now.  

1) Fish don't like the colder water, and it is reducing their metabolism, but otherwise are healthy.  Solution is to cover the tank

2) Something is wrong with the water that I have not found.  I need some other tests.

I would appreciate any input.  I have lost ~15 fish so far and have another corpse this morning.

Thanks

Bill

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Yeah, I've seen a lot of people use bird netting; it's relatively inexpensive.

Randall Wimbish said:

Although automatic feeders are not a good idea, it sound like your chemistry is good. Put some bird netting or blue Dow board over the tank. Or even shade cloth. You could possibly have an aggressive fish chasing them out. Shining a light in the tank will cause them to jump out also.

Thanks Alex and Randall.  I will be getting a mesh cover today.  I am still concerned that the fish in the tank are not eating.  

Would the reduced water temperature as we enter autumn cause that big a change in their appetite?  The fish in the water are staying at the bottom of the tank in a group, and seem to be beating their pelvic fins a lot (but I can't tell if it is more than normal).

Thanks

TILAPIA DO NOT DO WELL IN WATER COOLER THAN 74 DEG F.   

Jack - Thanks for the response.  From my reading the Nile tilapia are the best on cold tolerance, and the lower bound is 65 Deg F.  My experience was my fish thrived in much warmer water - in the 80s.  Sounds like I need to get my water temp up to the mid 70s

Bill

Bill,

I was wondering how many fish are left in the 180 gal tank and about what size.

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