Nitrates Low

[Jamie P Holcomb]

Quick question concerning the balance of all the things going on in my aquaponics. Lately my fish have stopped eating and it is starting to concern me. My parameter levels are pH-6.6, ammonia-.5, nitrites-.25, nitrates-5. Are these normal ranges for an aquaponics? I thought I had read that the ammonia and nitrites should be near 0 once the system is cycled. The system was cycled before I added fish where ammonia and nitrites were basically 0 and nitrates were at least 80. Since I added fish the nitrates have dropped significantly. Is there anything I need to be concerned about?

[Jim Joy]

Tilapia are OK with a few ppm of ammonia.  I have seen ammonia up to 8 ppm in my aquariums without the tilapia dying.  Not a good idea.  Less ammonia is better.  

My fish have survived 200 ppm nitrates.   As the bacteria reproduce and consume the ammonia and nitrites, nitrates will increase.  Plants will consume the nitrates.  So, media covered with bacteria keep ammonia down and plants consume the converted nitrates.  

I am running a 150 gallon tank with three 50 gallon media beds.  That is the minimum configuration of 1:1 water to filter (media beds) size. Water pump rate changed the water twice an hour.   My ammonia and nitrates were heading up fast.  I had to change water once a week to control the water quality.  Doubling the pump rate to change water four times per hour quickly reversed the trend.  Now ammonia is around .25 ppm with nitrates between 40 to 60 ppm.  The plants are growing rapidly even though it is summer in Phoenix AZ.  

PH is depressed by the natural acids produced by the bacteria.  My tap water is liquid rock with a ph of around 8.  The tank was running less than 6.  A net bag of washed oyster shells helps keep the ph up around 7.  Buy oyster shells at the feed store.  Ph affects the plants more than the fish.  However, fast changes in ph can kill most fish.  

So, your water chemistry sound OK.

Water temperature will affect how the fish eat.  Any other changes from when the fish were eating well?