In Aquaculture water quality is critical

Aquaculture water quality is the single most important factor in developing any aquaponic system. However, it is something many of us take for granted. We are so used to clean, clear water simply appearing whenever we turn on a tap, that seeing how quickly things can change comes as a bit of a shock.

Rainwater is certainly quite pure - when it falls - but what does it pick up from the things it falls onto? Not only that, but is 'Purity' really the thing we are looking for? Usually, rainwater is collected from a roof and this can be a dangerous thing for fish. Galvanized steel can add toxic amounts of Zinc to the water (toxic to fish that is), Colorbond, painted steel and tiled roof catchments can add potentially toxic organic chemicals from their coatings.

These are some of the reasons I chose to try and avoid having to make regular volume changes in my system, as a means of maintaining good Aquaculture water quality. Once the water has been conditioned, it is much better if you don't have to replace too much of it.

OK, so what water do you use? Well, tap water - conditioned tap water. That means water that has been treated to remove harmful things like Chlorine. This can be done quickly by using an activated charcoal filter, or slowly by aerating it for a few days (depending on the temperature) before using it. I do the latter, because I have better things to do with my time and money than fiddle about with lots of different filters.

Now you have a tank full of water that won't kill your fish, but can we now say that it has "Good Aquaculture Water Quality"? Sorry, no, not quite yet.

Fish require levels of salinity (salt), hardness (Calcium and Magnesium) and low levels of various nutrients like Phosphorus, Ammonium and nitrate. This makes aquaculture water quality very different from drinking water quality (which is just low in everything). Getting these things in or out as needed is the next trick. To overcome this problem, I developed a bio-filter that does the job fairly well and I also put some stuff into the tank before I added the water.

Actually, I had anticipated this and so the bottom of the tank has several large lumps of limestone sitting on the bottom. I actually chose this stone from a source that also contained some Magnesium (not enough for it to be termed a Dolomite, but this would work too). These started the process of increasing the hardness of the water while it was being conditioned.

I had hoped that this would give me good aquaculture water quality. Unfortunately, they were not enough and when I first added fish the water quality deteriorated to a green slime in a matter of hours! Of course, what I didn't anticipate was that good Aquaculture water quality also required a balanced biological component - algae and bacteria (but of course not too much).

So I diverted some of the flow from my aerators (I use a simple air-lift system to both aerate and generate some flow in the water) into a plastic bin set on the side of the tank. This had a simple gravity feed to let the water back into the tank again and let me experiment with different materials.

What I did was to cut the bottom out of a square plastic bucket and sit that in the bin, on top of four thin paving bricks standing side on. These kept the top of the bucket above the side of the bin and let me sit more limestone gravel on top (up to about half the depth of the bucket. Then, I managed to find a square aquarium net large enough to fit on top of the bucket and I could put other material inside this for my various experiments.

One material I tried was an expanded clay (that looked like orange-brown marbles) from the local hydroponic store. They recommended this as a possible plant growing medium for hydroponics, because it was light and let water flow through easily. Actually it was quite difficult to use at first, because it would float up and over the top of the bucket.

Once I got enough of it to sink and stay in the net (inside the bucket), I thought I had finally cracked the problem of maintaining good aquaculture water quality. The water became totally clear and looked great. Unfortunately, the fish didn't seem as impressed. I had stocked the tank with about $100 of large rainbow trout and the moment I started circulating water through this stuff they went crazy and started attacking each other. Within a few days they were all dead!

After having the clay medium analyzed I found the problem - absolutely mountainous quantities of Mercury!!! No wonder the water was so clear - nothing could live in it! Thank goodness I am not interested in hydroponics - I might have also been eating food grown in this CRAP!

Eventually, I found that Barley straw was the best stuff to help maintain good Aquaculture water quality. So here is how my bio-filter works now:

First, a simple elbow in the pipe returning water to the tank is enough to maintain some directional flow in the water. This seems to be calming to the fish, but also help circulate the particulate material and mix the water to help ensure the effectiveness of the filter.

The airlift pipe brings aerated water over the side of the tank and into the to of the bucket. This holds a couple of handfuls of barley straw in a fine net, which serves a number of purposes. The straw helps remove phosphorus and maintain the right algae-bacteria balance. Together with the net that surrounds it, many of the larger particulates are removed and the rapid flow of water out of the airlift pipe is slowed and diffused as it passes through the limestone pieces and into the bin.

It is now moving slowly enough that finer particles can settle out before the water flows back into the tank. Over time, algae also grows inside the bin to further remove unwanted nutrients. The net containing the straw is emptied into my compost bin on a daily basis and the bin is cleaned out weekly.

The last component of aquaculture water quality is plant growth to further remove nutrients. At present I use a covering of water fern (Azolla sp.) on the water in the tank itself. This is a Nitrogen fixer, so it mostly removes Phosphorus. However, it also responds to the Calcium in the water, giving a good indication of the hardness level, while also providing some 'shelter' that is also important for maintaining a low stress environment for the fish.

This sensitivity to Calcium can be seen in the comparison photo on the right. On the left is Water fern growing (actually slowly dying) on water that is not circulated through limestone. On the right is how it looks with good aquaculture water quality. Both photos were taken at the same distance and magnification.

Together, these things are just sufficient to maintain good aquaculture water quality in my 3,000 litre tank as a holding system. However, for growing fish or holding more than a dozen or so large fish, it is not quite enough. The addition of a small UV clarifier has helped (deliberately undersized as I want to control algae, not sterilize the water). So now I am working on an upgraded system. You can read more about this on my Projects page.

Lastly, there is the matter of volume changes. This system has prevented me having to do this on a regular basis, but it doesn't prevent the need to top-up the tank from time to time due to evaporative losses. Fortunately, I have an easy answer to this problem as well. The need for top-ups is greatest in hot weather and this is when we also have the evaporative cooler running in the house. When this is turned off at night, it dumps the water it has been recirculating. Rather than let this simply run down the drain, it is fed straight into the aquaculture tank and seems to be enough to maintain the volume with no other additions. It has also been preconditioned by circulating past a fan all day, so there are no ill-effects on the fish.

We now have a reasonably good idea about maintaining good water quality, but the learning curve remains a steep one. More information will be added as we get it and there are some useful links below. You can also subscribe to our Free email newsletter - The Food Gardener.

Further reading and Useful links

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