Aquarium water, fact or myth?

I keep tropical fish and for all my time keeping plants (only a couple of years up to now), I have always used aquarium water to water my plants, along with some other feeds too.

Any forum you go on and people gush over the benefits of aquarium water as a top quality feed for your plants.

Anyone who keeps tropical fish will understand that weekly water changes need to be performed because the end cycle of the nitrogen cycle in an aquarium is nitrate, which can have detrimental effects on fish in the long term.

When we do our water changes the natural place to flush the old water is down the drain, but not if you have plants. The nitrate in the old water is a good fertiliser for plants so it is good for us aquarists, who are also plant parents, to store some of this old water as feed.

And this is no doubt what countless thousands of aquarists/plant parents do up and down the country.

I was thinking only the other day what type of nitrate levels are there in old aquarium water compared to a diluted off the shelf typical plant fertiliser from the garden centre. So I did an experiment, and was quite shocked.

My nitrate levels in my old aquarium water never exceed 40ppm, as measured by the API aquarium water test kit. As mentioned, nitrate can have adverse long term effects on aquarium fish so I don't let it build up too much. I keep on top of it via my weekly water changes. This 40ppm reading equates to an orange colour on a colour chart. The colour chart ranges from bright yellow (0ppm nitrate), through orange to red to deep burgundy. Each change in colour represents an increase in nitrate. The burgundy is around 160ppm. This wouldn't be pleasant for fish at all, no aquarist in their right minds would run their tanks at such high levels.

When you do the 5 minute test it always starts off bright yellow. Then gradually starts turning colour until it settles on whatever the level of nitrate is in the water.

When I tested the off the shelf plant fertiliser (miracle grow) diluted in water as per the instructions, I was flabbergasted at the nitrate level. It immediately bypassed the yellow, orange and red spectrum of the test card, and went more or less straight to dark burgundy and beyond! The nitrate measurement was substantial, at a guess into the many hundreds ppm.

No wonder plants benefit from being fed liquid fertiliser!!

What this tells me is that aquarium water, although some swear by it as a feed, is nowhere near as concentrated as a dedicated plant feed. In fact you could say that there is very little nutrient in aquarium water at all. Maybe aquarium water is like a placebo effect. We think it's great because plants are thriving, though in reality they are probably thriving for umpteen other reasons rather it being the result of being watered with old tank water.

Very interesting, not to mention eye opening.

 

1% nitrate = 10000 ppm.
Not sure plants are subject to placebo effects, you believe that something will help then the condition improves.
It could be that you are paying more attention to the plant so you catch problems earlier; a bit like talking to them.

 

If that is correct then it proves a lot. My aquarium water at 40ppm, and probably thousands of others too, can't even be making the smallest difference to plants.

Maybe it's nothing to do with the nitrate why plants do ok, but more to do with the fact that aquarium water doesn't have chlorine in it either?

 

I kept tropical fish for around 50 years.
I used one of those syphon vacuum tubes to pull the gunk out of the gravel and used the 'dirty' water for plants inside and outside.
It is useful for plants, but it is not a fertilizer in any way.
Rather, it's a bit like weak seaweed extract. It contains tiny amounts of all sorts of stuff that are beneficial for plants.

For a while, I had a CO2 injector and twin 150w halide lights.
Every week I'd pull out a bucket load of plants as they grew so fast and they went on the compost heap.
In the pic above you can see oxygen bubbles coming from the plant leaves.

I stopped keeping tropical fish about 5 years ago when I had my house refurbished.
The effort to keep the tank pristine was getting too much and the floor was getting too wet!

 

Slightly off track but I have some Spanish moss hanging around in the garden.
I often dump it in my pond for half an hour and it seems to thrive on it.

I know ponds are different to fish tanks but the principle is the same tiny amount of nutrient must be available to plants.

 

This is exactly how I'm beginning to look at it. Still good for plants like you say, especially those which are softies when it comes to chemicals in tap water, such as the prayer plant family, which I have many.

 

Filter and substrate gunk is the benefit of a fish tank. If you remove the detritus and poo before it has had a chance to break down then it is far more beneficial to plants.
I think you will also get conflicting views because if you grow a higher tech planted tank with added fertilisers then there is more in the water to benefit plants. Many planted tank people use the EI (estimated index) method of fertilisation which is essentially chuck enough in to get your plants to grow strongly without any deficiencies and then do a massive water change every week to reset it so nothing is raised to dangerous levels.
Plants also seem to love aquarium water and many grow plants out of the top because they have access to atmospheric co2, which works in even no tech tanks, but you sometimes see slow growth is there isn't the right balance of nutrients in the tank water. It is the balance we need not just nitrate.

 

This is something I also do. Although I do my water changes weekly, I also do bi-weekly filter sponge rinses. The water rinsed from the sponges is dark with fine particulate.

I store this "muddy" water in a large tub. When the particulate has settled at the bottom I slowly pour off the clearer water from the top.

Over time I am left with dark and extremely smelly sludge. And boy, when I say smelly, I mean sewer smelly. I suspect this is the "rocket fuel" type fertiliser.

The problem with this is I can't readily feed it to my plants because over time if I surface feed the soil all the fine particulate clogs up the surface layer, affecting quality of drainage, which of course is key for any plant.

Yes, that sludge I collect is a tricky one, still trying to find the best way to utilise it really, but there's no doubt it will be far more beneficial that just plain old aquarium water.

 

Certainly it makes sense that aquarium water is better for plants than tap water- full of micro- organisms, much less chlorine, usually quite soft plus at room temperature. Win win.

The sludge sounds like it demands to be forked in the soil around roses, @Esoxlucius . Lush houseplants would probably love it but the smell might not be the best in a living room, a bit like fresh chicken pellets.

Vaguely on topic, years ago my Sister moved into a flat on fourth floor of a lovely old building in Helsinki. There was no lift and the magnificent staircase was spiralling type without landings.

She paid for a removal firm instead of roping in family and friends to move her stuff- for a reason. She had a 800 litres aquarium

Pity this was before home videos time as it must have been a sight getting the two-bathtub sized fragile glass tank up those stairs.

 

@Esoxlucius
While you are in experimental mode, may I suggest you try this. Take a volume of the Miraclegro solution and dilute it until it records the same level of nitrate as that of an average tank .This will give a ratio of nitrate for your plant use of the waste water. It may also give a clue as to the honesty of Miraclegro as to how much of their product one would actually need to satisfactorily maintain a plant, rather than the amount they would like you to use...
Rather than waste the water, perhaps use it to dilute the fertiliser to a resonable strength, according to observed results. I would hope that the fish waste contributes more trace elements etc as well.

 

I also find the sludge forms a crust on the surface of indoor pots so it's exclusively used in the garden where its gets washed down easier. If I was less lazy I might dry some out and make my own fertiliser tabs for use indoors, like my friend does with his guinea pig poo (he also uses them as root tabs for his aquatic plants).

 

Have seen photos from friends in South Africa where they have set up an intergated hydro system starting with very large tank ( about 20ft diameter) for a good few medium sized fish and the water is then circulated around their hydro crops in a large polytunnel , then back into the fish tank.

It seems to work for them, but using our typical domestic fish tanks water and sludge on our food crops, not too sure ?
Doubtless be fine for other ornamental plants but our tanks substate can be home to a variety of organisms like Planarian so would not want to eat anything exposed to such things, even if the risk is low.

 

When I kept fish, I experienced regular nitrite spikes and no matter what I did, I couldn't stop them.
That was until I rigged up a trickle filter system. Ugly as sin, but the water quality was fantastic. Plant growth was lush and colours from shoaling fish like neons, harlequins and cardinals seemed accentuated. Some one asked me to house their barbs....they trashed the place...I hate that person!
Anyway, I digress, all water changes watered the garden and house plants...they seemed to enjoy it

 

Many moons ago I both kept tropical fish and had a job selling them. In neither situation was water changed every week. The tanks all had under gravel filters so the droppings were drawn down into the gravel where they were broken down and used by the plants. Tanks were cleaned using a sort of mini vacuum which was actually an airlift which deposited excess crud into a bag.
Modern filter systems should make it even less necessary to do regular water changes.
Fish all stayed healthy and I actually bred some more unusual ones at home, so they must have been happy with the water condition.

 

This idea piqued my interest, so I did it! The results are staggering really, but I'm not sure how to interpret them. Here goes.....

The recommended dose of Miracle-gro is half a cap per 1 litre of water. This half cap measurement is exactly 15ml. This is what I did initially and the result was a very very deep burgundy colour on my chart almost instantly. The nitrate level must have been in the many hundreds, even thousands ppm. This is the level of dosage which the expert fert manufacturers have come up with.

Then I did a drastically reduced Miracle-gro addition. In fact I added only 0.625 ml, which is one twenty fourth of the recommended 15ml dosage. The colour in the test vial went through yellow, orange and red pretty quick. It slowly settled on the burgundy colour which is around 160ppm nitrate. In aquarium terms this would equate to water which was well past it's water change time. Some would argue this level of nitrate is wayyyy too much for fish.

So basically what that quick test showed me is that even at a dosage of Miracle-gro at one twenty fourth the recommended level, the level of nitrate is still way way higher than an average aquarium with a nitrate level of around 40ppm. Way way higher.

Does that mean that we can all drastically reduce our Miracle-gro usage and make one bottle last literally a lifetime? I'd say certainly not. My test is for nitrate only. There are loads of other "goodies" in the Miracle-gro which aren't found in aquarium water.

I think it would be safe to say, as others have already said, that aquarium water is a good chlorine free "filler" which can safely be used in conjunction with the proper tried and tested off the shelf ferts. But stating, as some do, that aquarium water is the best thing since sliced bread, then I believe this to be a little far fetched.