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I put 13 tilapia into the fish tank (300 gal) 3 weeks ago (added 4/14/2012). They thrived doubling in size some up to 8oz and the size of the palm of my hand. Now I am pulling out dead fish daily. Most caught before floating but two had been in the ecosystem long enough to begin floating.

Fish tank is 300 gal, sump is 150 gal,  two grow beds 60 gal.  PH is between 7.4 - 8. Temps have hovered between 65f - 73f

Plants in grow bed are extremely vigorous and growing at accelerated rate. 

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I would start by checking you ammonia, nitrites, and nitrate levels with the basic aquarium test kit or quick dip sticks. By what you have said I would bet ammonia levels are high. Good Luck and when in doubt water change! 10-20%.

I have slightly over populated my karp colony and I can tell when the water is bad just by the way they act.  When I have checked parameters when the red flags go up my ammonia and or nitrites have been the issue. I water change and also added more aeration. My system water volume is just over 200 gallons. Also, when I was first getting my colony healthy I had alot of die off. I figured this was because the quality of fish I was getting was poor. Quality of breeding conditions could also be a factor. Mass breeding of fish can lead to poor health later on. Sometimes bad health of fish takes time to surface.


 
Jonathan Farrand said:

I would start by checking you ammonia, nitrites, and nitrate levels with the basic aquarium test kit or quick dip sticks. By what you have said I would bet ammonia levels are high. Good Luck and when in doubt water change! 10-20%.

I second Jonathan's recommendation,

We need to know the results of the water tests for ammonia, nitrite and nitrate as well as temperature and pH before we can really venture much of a guess about what is going on.

Also, How are you pumping?  Continuous with siphons or timed flood and drain?  Are you moving that 300 gallons of fish tank around each hour or just a fraction there of.  With a new system I'm guessing you are suffering from new tank syndrome since the bio-filter is only just starting to cycle up so your fish could be dieing of ammonia and/or nitrite being too high.  Extra aeration, and stop feeding if the ammonia and/or nitrite are over 1 and if it is higher than that you may need to be doing some partial water changes to keep from killing more fish though if things are too bad, the damage may already be done but tilapia are tough and maybe you will be lucky enough to save some of them.

Good Luck

I had the same exact problem and never learned what was happening. It started happening after 4 weeks. Here are the theories: too much crud in the fish tank (I had little bits of stuff coming from the grow beds and the water got very dirty). I put stockings over the siphon pipes under the garden beds and another on the water coming into the fish tank. Bullying. We did find a couple of fish that were systematically beating up on the others and one was dying per day. I did find a fish that was doing that and removed it and that helped somewhat but didn't completely solve the problem. Overfeeding. This turns out to be impossible but if you have food collecting on the bottom you should remove it. 

Your pH is a bit high and you should add a buffer to bring it down. Also your temps are a bit cool but shouldn't kill fish. 

All my chemicals were fine -- ammonia, nitrite, pH etc. I'm using rainwater from a cistern so it's nothing chemical like chlorine. I finally had to clean out the entire tank and start over. I'll be interested to find out what else you hear. 

Michael,

A BUFFER Will NOT bring pH down!!!!!!  Buffers like calcium carbonate or potassium bicarbonate will elevate pH.

And a pH of between 7.4-8.0 is just fine for cycling and only a concern if ammonia is high.  The plants might prefer the pH to be lower but the fish are just fine with the higher pH so my advice is to NOT MESS WITH IT.  The Less messing with your water chemistry you do the better.  Any messing around you do in a new system tends to slow cycling

As an an aquarium and planted aquarium keeper first I agree 100% don't over think this issues with aquaculture. The natural build up of the eco system takes time. Sometimes more time than what we want. Many begining aquarium keepers have issues cycling their tanks and go out and buy all types of chemicals and treatments. I did for sure! But now after keeping fish for over four years(aquaponics one and a half years) I say let mother nature run her course.

The microbes thrive on oxygen and produce faster or slower depending on temperature. A rule of thumb I have with oxygen is you can never have too much in a system.  I even like to have an air stone under my small float bed. I do try to always consider gravity and how that can help me add oxygen while water is getting pumped or returned to the beds and tanks.

 

Ooops. Right. Not a buffer. Got my up and down confused. 

OK Test results are in;  Ammonia is > 8ppm , Therefore I did an emergency transfusion of about 150 gal. Ph is 7.3 and I do think I had a bit of crud due to over feeding.  My system is constant pump from sump tank up to grow beds and fish tank simultaneous. Approximately 50 gallons per hours turn over in the fish tank.

  In addition to the transfusion I lifted a bit of the crud, it had a bad "dead smell" to it.  I will test again in a few hours.

The new water (city tap) was held in poly drums for 48 hours, so no chlorine there. I am storing new water as I type. I was told  that 1000mg of Vitamin C will immediately de-chlorineate the water, any information on that fact?

Don't know about the vitamin C additive but it does sound interesting. I usually just fill my buckets and let them sit for a day or two before I add them to the aquariums or the aquaponic system.

You have done your work by water changing. You can from here do small water changes daily to help combat the build ups, that will help reduce your bio-filter stress load by physically removing ammonia,nitrites, and nitrates. That method is done often with aquarium keepers. Discus keepers when breeding may do heavy water changes daily to keep the nitrogen cycle levels to a zero at all times. You need time for your bacteria and microbes to flurish and water changes to keep your fish alive to me really is the most straight forward and basic way to save the fish.

In my experience with fresh water planted aquariums, tropical fish keeping, ponding and now aquaponics the longer the systems are going the more amazing they become!

 

If you can add a bubbler to your prep water it will help blow off the chlorine (provided your tap water is only treated with chlorine and not chloramine and no I don't know about the Vitamin C as I'm on well water.)

For your 150 gallon tank, I would really recommend you pump at least 150 gallons of water through it per hour, especially while cycling up.

Ammonia would definitely be a likely cause of your fish deaths.  New system and overfeeding are both causes.  DON'T FEED AGAIN until the ammonia and nitrite both drop.  You might want to prepare for the nitrite spike that is coming next, here is a blog post about salt.

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