Backyard Aquaponics
A place to discuss aquaponics

We started our systems (2 IBCs) in late April (2014) and had initial nitrite spike after adding our fish; 25 fingerling tilapia. The spike resolved in about 2 weeks, and we didn't lose any fish. The system has been doing pretty well, despite the summer heat in Texas. Our set-up is in a greenhouse with ventilation by fan. In late August we started having a trace of ammonia; then also a trace of nitrite. We decreased our feeding a little; the fish eat all the food in seconds. The ammonia and nitrite have continued to go up and are as follows:

Last week:
Ammonia: 0.25
Nitrite: 0.25

Today's test is:
Ammonia: 0.50
Nitrite: 0.50

pH is 6.8-7.0
Nitrate is 160ppm, and hasn't decreased at all, even though the grow beds are FULL of healthy plants. Water temp ranging from 78-84 degrees F. The system is a flood and drain with the only filtration being done by the grow bed.


What could be the cause of the gradually increasing ammonia and nitrite? And what should we do about it? We have something to treat the ammonia, but not the nitrite. I've read that it is suggested to stop feeding the fish...they are now between 4 and 7 inches on average. We are feeding 1 1/2 tablespoons pellets in am and 1 tablespoon in evening.

What is the total WET gravel volume of your GB's?... ie: GB length x width x maximum water level in gravel.

What type of Flood & Drain, siphoned or timed?... and what are the F&D cycle times?... how long to flood, how long to drain?

Is there a build up of waste on the floor of the FT or ST?

Do you have any pics of the system, in particular the GB's?

The grow beds are 3'2"X3'9" with about 5 inches of hydroton; 4 bags (40L) each . One tank stays a bit wetter than the other; so the top stays wet/moist and the other is mostly dry.

It is a timed F& D cycle set at 15" flood and 45" drain/off.

I don't see any build up of waste on the floor.

I am having trouble with the pics; too big, even on my smallest setting. I'll try another camera......

Flooding the gravel all the way to the surface is not ideal, most systems have their max flood level at least an inch below the surface to avoid excess evaporation, algae build up, and rotting of seedlings etc. In that situation you would have 96 x 114 x 10cm of wet gravel, giving you a total of 100-110L of wet gravel per GB... so each GB would effectively be able to support 5x Tilapia through to plate size... maybe a few more when they are smaller, but as they grow so will Ammonia and Nitrite issues, as you are seeing.

You are way overstocked, you need more GB's, and more wet gravel. Shallow GB's are not recommended, 300mm (12") is widely accepted as a good depth.

P.S. Resize your pics to 800x600 using ACDsee or Microsoft Office Picture Manager, or similar.

Flood and drain is the cause of your problems.

How so?... Flood & drain in general, or the method/timing being used?

So increasing the grow bed material will increase the surface area for the nitrifying bacteria, right? We are in prep stages for adding two more tanks/grow beds and plan to thin out the fish that way when they're ready. We've seen systems that support more and bigger fish than ours, but they had an additional filter. Any suggestions on what kind of supplemental filter might be useful?

Also, I would like more detail on the flood and drain comment as well....not exactly sure what you mean by that being the cause of our problems.

Thanks so much for your input! It's great to have some other people with experience with aquaponics available for assistance.

More gravel filled GB's!... preferably with 300mm of gravel. They act as both bio-filters and mechanical filters.

If you really want to go down the extra mechanical filtration route, then a radial flow filter is probably the best solution. Removing the fish waste solids from your system will also remove a good deal of the nutrients, especially trace elements, so you would need to look into a mineralisation tank to reclaim at least some of those nutrients.

There are many very experienced AP'ers that have been growing for years without supplementary mechanical filtration in their systems and achieving outstanding results... It's just matter of designing the system well, getting the system balanced, and running sensible stocking rates.

In my experience - timer based flood and drain systems don't exhibit great biological filtration qualities. I believe it's from the fact that the media is usually only in water 1/4 of the time. During the rest of the time the bacteria quickly convert the ammonia & nitrite in the thin film of water left and end up sitting idle for most of the time.

It doesn't seem to be as much of a problem in syphon systems. I think that's because of the quicker cycle times.

So if we increase the cycle time, that may help in getting the ammonia and nitrite down? That's interesting, because about 2-3 weeks ago I decreased the cycle time back to 15 min flood, 45 min drain, because it looked like the plants were too wet. I had it set previously at 15 min flood and 30 min drain through the summer because it looked like the plants needed more water with the 98-100 degree temps.

Yeah, increasing the amount of time the media is wet will certainly help. Have you considered running constant flood? Set the water level an inch or two below the surface of the media?

When you say the plants were too wet, what do you mean? Is your water level really high?

Look at a DWC system, the roots are submerged in water for the plants life. They can handle it very happily.

Maybe just run it without the timer, the more you cycle the water though, the more the filtration/bacteria will convert the ammonia.

We did have it on constant for a while....the plants started dropping leaves; I think the water level is/was too high. We adjusted it as much as possible at the time. We need to add more drains so it doesn't flood so high. (Not easy to do when the bed is completely planted....) We probably also need more gravel, as suggested. We've also changed the timer to cycle more often. I haven't seen a DWC system, but i'll check it out. Thanks.

Don't add more drains - what size is your standpipe ? Just adjust the inflow

Overstocked.