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I have a mixed media system that's been up and running for about 2 or 3 months now.  The system is powered by a fish tank of about 375 gallons that holds about 20 tilapia and few cichlids.  I would put the combined weight of the fish around 15#, but probably less.  The fish are behaving normally


The water is pumped through a gravel bed of about 175 gallons or river rock (which does have some worms in it, but not a whole lot) and then passes into my raft bed which holds another 120 gallons of water, and then back to the FT. Both beds are heavily planted with basil, oregano, peppers, a few small eggplants, cherry tomato and a tatume squash.  The plants look really good but I have added maxixrop with iron twice, the last time being about 2 weeks ago.


Recently, my water specs have risen some and Im trying to pin point the cause as the system has not changed about 6 weeks.  The specs are as follows:


Ammonia .25ppm


Nitrate >80ppm>160ppm

ph 7.4-7.6


After the initial cycle, the ammonia and nitrites were at 0 up until a few days ago and the nitrate was around 60-80.


I'm thinking that I maybe have over-stocked and now the bacteria is havin a hard time keeping up with the waste. I dont see an over accumulation of solids in the bed (at least I dont think so), but there are some for sure.  The roots in the raft bed look pretty clean, some gunk but not too much.  Im considering adding a swirl filter to help remove solids BEFORE the gravel bed, but Im concerned that that kind of defeats the point of the bed.  I realize that the swirls are generally incorporated after the water passed through the media bed.


I know the specs aren't really off the charts at this point, but I wanna try and nip the water issue in the bud before it becomes a real problem.


Any ideas?




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Is there any build up of feed or fish waste in the fish tank? 

Have you inspected for dead or dieing fish hidden somewhere in the fish tank?


How much water are you pumping per hour?

Do you have supplemental aeration?

What are the water temperatures?


What feed are you using?


Any of these things can affect the load on the system.  If the temperatures are staying over 80 F overnight and you don't have supplemental aeration and are not pumping more than the bare minimum per hour, dissolved oxygen may be getting low enough to slow down the bacteria.


Or perhaps you are just nearing the limitation of your grow bed and aeration levels for your fish load.  The protein % and quality of the feed can really affect this too.  I've had old high protein feed cause water quality issues in my systems big time.  High protein feed for the smaller fish goes bad quickly too.

Thanks for the reply.  I have checked for dead fish and havent found anything, although I do have an issue with frogs laying eggs in my rafts beds.  Thus there are some tadpoles in it, but I havent seen any dead one and I remove the ones I can catch.  Im in the process of frog-proofing the system...


Im pumping about 1000 gph if not better, although there is no additional aeration apart from the return water to the FT.  I don't have an exact temperature at the moment, but being in the Florida Keys i know that my night temps are most always above 80 degrees...


As for feed, Im using a DIY feed that is comprised of leafy greens, carrots, sweet potatoes, some commercial feed, and local caught ballyhoo and BSF larvae.  Everything is pureed, mixed together with some gelatin and frozen.  It holds together really well and I dont see much of it or anything else for that matter built up in the ft. I normally feed 2x a day, but since the water chemistry thing, I've gone down to once per day.


Maybe I am just maxing out my growbed.  I'll add some aeration and see how that effects things.  


What about my comment on adding a swirl filter before the gravel bed?  Counter-productive?



Is your media grow bed constant flood or flood and drain?


1000 gph is a good amount of flow.  How much disturbance to the water do you get from the water return to the fish tank?


Some means of removing solids could reduce the dangers of ammonia and nitrite levels rising as long as you are removing those solids daily.  I would probably just recommend adding another media grow bed to provide more filtration as being a more productive way to go.

Flood and drain, one hour on, 30 minutes off.  A decent amount of disturbance, but always room for more.


Yeah, the second media bed is in the works.  Hopefully that'll do the trick.

I expect so.

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