Aquarium water as fertiliser

Hi guys,

This is my first post. I am a keen aquarist with a 360g tropical fish tank. I am a member of several fish keeping forums and a question that comes up regularly is, "Is aquarium water alone suitable as a fertiliser for house plants?"

I store my used aquarium water in 5g tubs, and I have lots! I also squeeze my filter media out into the water, which is full of gunky goodness for plants. The end product is a gunky smelly solution that smells very much like tomato feed you can buy in the shops.

I'm led to believe that old gunky fish tank water is about as good as you can get for plants, and don't get me wrong, my house plants are doing ok, though I think some could do better.

My question for you guys is, do you think that old aquarium water is as good as people say, or is there a chance it is missing several critical trace elements that would boost my houseplants further.

Should I use it in conjunction with proper houseplant specific ferts?

Thanking you guys in advance for any wisdom on this subject.

 

I saw something the other day where they had a greenhouse setup which involved fish in a large tank and the water being used to grow plants.

I dont know much about the chemical side of things but I think from memory, regarding my garden pond, something about nitrates and nitrites.
And it has to go through a biological filter before the fish waste can be used by plants.

 

Yes, it's called the nitrogen cycle. Anything in an aquarium that can decay (dead fish, uneaten food, fish waste, dead plant leaves etc etc etc) releases ammonia, which is toxic to fish. But the biological filter, which houses all the beneficial bacteria, converts the deadly ammonia to deadly nitrite, which in turn is converted to not so deadly nitrate. So basically, an healthy, mature aquarium only has nitrate in it because the toxic ammonia and nitrite are pretty much converted straight away.

The nitrate, over time, builds up, and that is why fish keepers have to do partial water changes to keep the nitrate build up in check. It can cause long term health issues in fish if it is allowed to build up to dangerous levels.

It's this nitrate build up in the water we change out which is why old fish tank water is good for plants. After all, fertilizers are full of nitrates.

But of course plants need other trace elements too, and since I have no way of testing my aquarium water for trace element/mineral content I thought someone might be able to shed some light on the matter.

 

OK, not really that well up on the subject.
Plants grow well in my garden pond without any added trace elements, but that is as far as I know.

 

I always used to use the water from my tanks when I kept fish, never found any harm to the army of houseplants and massed ranks of summer bedding. I did, however, install some McGyver type bio trickle filters in most of the tanks which could've neutralised nasties via the algal growth in them.
FYI don't mix barbs with heavily planted tanks...in ends up looking like a teenagers room!

 

The aquarium is also usually heated in the winter months to "room" degree and also acts as a temperature moderator. Kinda like have barrels of water in the green house to absorb heat during the day/release at night.

 

Lol. Yes, I'm well versed on fish related stuff. I've kept dozens of barb species in my years, and very few are plant friendly! But my interest in house plants and gardening is pretty new to me.

I'm very quickly finding out that keeping plants healthy is very similar to keeping fish in as much that certain species need a set of specific conditions before they can thrive. Lol.

 

Hi,

Afraid we cannot add that much more than you already know, and yes cannot see any reason why the water cannot be used on all plants as it contains plenty of nitrate (N).
It would be perfect as a summer lawn food/water.

However though containing some phoshates (P) think the level in garden plant fretilizer terms will be quiet low, assuming you are not using a phospate remover in your tank ?

The other key element is potassium (K) which is needed in the garden help produce good flowers and fruit, though again no idea what amount is in the waste water

Apart from the lack (?) of P & K, many other more specialised plants are often given extra elements eg Roses, so would think alternate waterings/ feeding with a typical plant fertilizer and then your tank water would be the best route ?

Think to get a more accurate answer the folk in the Planted Tank forum seem really into the specifics of fertilisers used and will be eager to clarify things in detail, assuming you are not in there already ?
UK Aquatic Plant Society

There are a few folk here in this forum who do use Hydroponics so they may be able to help as well .

 

As you say, the water would contain nitrate. When I kept Koi, it was always a bit of a battle keeping the nitrate levels down, resulting in lots of stringy algae. Seems to me that it should be pretty good for leafy growth. I do though wonder if it would be sufficient on its own? I guess I’d look into specific plant requirements and make a judgement about adding other elements/supplements.

 

Thanks guys for the replies. I knew that aquarium water was good but, as mentioned, it may be lacking in crucial trace elements. And without a huge fancy laboratory to hand I have no means of testing what those missing trace elements are!

I think moving forward I will supplement my aquarium water with a few drops of an off the shelf general plant fertiliser, which of course there seems to be thousands of, lol

For just general houseplants, some of whom flower once in a while, what would be your recommendations for a good fert?

 

Not sure if they would work purely on water, but you can get soil testing kits that give you some idea of the 3 main elements available in soil NPK, and roughly how much of each is present.

 

For plants in the house we get good results with the standard bottle of Baby Bio, easy to use and a bottle last us a whole year.

For your aquarium water, we would just use it on the lawn in summer and other large shrubs, but for fruits, veg and more expensive flowering plants we would stick to the usual garden fertilisers as the aquarium water might cause too much soft leafy growth.